What is Assumpsit?

Legal Definition
Assumpsit ("he has undertaken", from Latin, assumere), or more fully, the action of assumpsit, was a form of action at common law. The origins of the action can be traced to the 14th century, when litigants seeking justice in the royal courts turned from the writs of covenant and debt to the trespass on the case.

Throughout its long history, the action of assumpsit has been used to enforce what are now referred to as obligations arising in tort, contract, and unjust enrichment. Most significantly, it is out of the law relating to the action of assumpsit that the modern law of contract and of unjust enrichment emerged in the 19th, 20th and 21st centuries.
-- Wikipedia
Legal Definition
Contracts. An undertaking either express or implied, to perform a parol agreement. 1 Lilly's Reg. 132.

2. An express assumpsit is where one undertakes verbally or in writing, not under seal, or by matter of record, to perform an act, or to pay a sum of money to another.

3. An implied assumpsit is where one has not made any formal promise to do an act or to pay a sum of money to another, but who is presumed from his conduct to have assumed to do what is in point of law just and right; for, 1st, it is to be presumed that no one desires to enrich himself at the expense of another; 2d, it is a rule that he who desires the antecedent, must abide by the consequent; as, if I receive a loaf of bread or a newspaper daily sent to my house without orders, and I use it without objection, I am presumed to have accepted the terms upon which the person sending it had in contemplation, that I should pay a fair price for it; 3d, it is also a rule that every one is presumed to assent to what is useful to him. See Assent
-- Bouviers Law Dictionary
Legal Definition
Remedies, practice., A form of action which may be defined to be an action for the recovery of damages for the non-performance of, a parol or simple contract; or, in other words, a contract not under seal, nor of record; circumstances which distinguish this remedy from others. 7 T. R. 351; 3 Johns. Cas. 60. This action differs from the action of debt; for, in legal consideration, that is for the recovery of a debt eo nomine, and in numero, and may be upon a deed as well as upon any other contract. 1 h. Bl. 554; B. N. P. 167. It differs from covenant, which, though brought for the recovery of damages, can only be supported upon a contract under seal. See Covenant.

2. It will be proper to consider this subject with reference, 1, to the contract upon which this action may be sustained; 2, the declaration 3, the plea; 4, the judgment.

3. – 1. Assumpsit lies to recover damages for the breach of all parol or simple contracts, whether written or not written express or implied; for the payment of money, or for the performance or omission of any other act. For example, to recover, money lent, paid, or had and received, to the use of the plaintiff; and in some cases, where money has been received by the defendant, in consequence of some tortious act to the plaintiff's property, the plaintiff may waive the tort, and sue the defendant in assumpsit. 5 Pick. 285; 1 J. J. Marsh. 543 3 Watts, R. 277; 4 Binn. 374; 3 Dana, R. 552; 1 N. H. Rep. 151; 12 Pick. 120 4 Call. R. 461; 4 Pick. 452. It is the proper remedy for work and labor done, and services rendered 1 Gill, 95; 8 S. & M. 397 2 Gilman, 1 3 Yeates, 250 9 Ala. 788 but such work, labor, or services, must be rendered at the request, express or implied, of the defendant; 2 Rep. Cons. Ct. 848; 1 M'Cord, 22; 20 John. 28 11 Mass. 37; 14 Mass. 176; 5 Monr. 513 1 Murph. 181; for goods sold and delivered; 6 J. J. Marsh. 441; 12 Pick. 120; 3 N. H. Rep. 384; 1 Mis. 430; for a breach of promise of marriage. 3 Mass. 73 2 Overton, 233 2 P. S. R. 80. Assumpsit lies to recover the purchase money for land sold; 14 Johns. R. 210; 14 Johns. R. 162; 20 Johns. R. 838 3 M'Cord, R. 421; and it lies, specially, upon wagers; 2 Chit. PI. 114; feigned issues; 2 Chit. PI. 116; upon foreign judgments; 8 Mass. 273; Dougl. 1; 3 East, 221; 11 East, 124; 3 T. R. 493; 5 Johns. R. 132. But it will not lie on a judgment obtained in a sister state. 1 Bibb, 361 19 Johns. 162; 3 Fairf. 94; 2 Rawle, 431. Assumpsit is the proper remedy upon an account stated. Bac. Ab. Assumpsit, A. It will lie for a corporation, 2 Lev. 252; 1 Camp. 466. In England it does not lie against a corporation, unless by express authority of some legislative act; 1 Chit. PI. 98; but in this country it lies against a corporation aggregate, on an express or implied promise, in the same manner as against an individual. 7 Cranch, 297 9 Pet. 541; 3 S. & R. 117 4 S. & R. 16 12 Johns. 231; 14 Johns. 118; 2 Bay, 109 1 Chipm. 371, 456; 1 Aik. 180 10 Mass, 397. But see 3 Marsh. 1; 3 Dall. 496.

4. – 2. The declaration must invariably disclose the consideration of the contract, the contract itself, and the breach of it; Bac. Ab. h. t. F 5 Mass. 98; but in a declaration on a negotiable instrument under the statute of Anne, it is not requisite to, allege any consideration; 2 Leigh, R. 198; and on a note expressed to have been given for value received, it is not necessary to aver a special consideration. 7 Johns. 321. See Mass. 97. The gist of this action is the promise, and it must be averred. 2 Wash. 187 2 N. H. Rep. 289 Hardin, 225. Damages should be laid in a sufficient amount to cover the real amount of the claim. See 4 Pick. 497; 2 Rep. Const. Ct. 339; 4 Munf. 95; 5 Munf. 23; 2 N. H. Rep. 289; 1 Breese, 286; 1 Hall, 201; 4 Johns. 280; 11 S. & R. 27; 5 S. & R. 519 6 Conn. 176; 9 Conn. 508; 1 N. & M. 342; 6 Cowen, 151; 2 Bibb, 429; 3 Caines, 286.

5. – 3. The usual plea is non-assumpsit, (q. v.) under which the defendant may give in evidence most matters of defence. Com. Dig. Pleader, 2 G 1. When there are several defendants they cannot plead the general issue severally; 6 Mass. 444; nor the same plea in bar, severally. 13 Mass. 152. The plea of not guilty, in an action of assumpsit, is cured by verdict. 8 S. & R. 541; 4 Call. 451. See 1 Marsh, 602; 17 Mass. 623. 2 Greenl. 362; Minor, 254 Bouv. Inst. Index, h. t.

6. – 4. Judgment. Vide Judgment in Assumpsit. Vide Bac. Ab. h. t.; Com. Dig. Action upon the Case upon Assumpsit; Dane's Ab. Index, h. t.; Viner's Ab. h. t.; 1 Chit. Pi. h. t.; Petersd. h. t.; Lawes PI. in Assumpsit the various Digests, h. t. Actions; Covenant; Debt; Indebitatus assumpsit; Padum Constitutiae pecuniae.
-- Bouviers Law Dictionary
Legal Definition
Lat He undertook; he promised. A promise or engagement by which one person assumes or undertakes to do some act or pay something to another. It may be either oral or in writing, but is not under seal. It is express lf the promisor puis his engagement in distinct and definite language; it is implied where the law infers a promise (though no formal one has passed) from the conduct of the party or the circumstances of the case. In practice. A form of action which lies for the recovery of damages for the non-performance of a parol or simple contract; or a contract that is neither of record nor under seal. 7 Term, 351; Ballard v. Walker, 3 Johns. Cas. (N. Y.) 60. The ordinary division of this action is into

(1) common or indebitatus assumpsit, brought for the most part on an implied promise; and

(2) special assumpsit, founded on an express promise. Steph. Pi. 11, 13. The action of assumpsit differs from trespass and trover, which are founded on a tort, not upon a contract; from covenant and debt, which are appropriate where the ground of recovery is a sealed instrument, or special obligation to pay a fixed sum; and from replevin, which seeks the recovery of specific property, if attainable, rather than of damages.
See also
-- Black's Law Dictionary
Legal Definition
Derived from Latin assumere, meaning to assume or undertake. In contracts, it is understood as an undertaking, either express or implied, to perform a parol agreement. At common law, it was divided into express and implied assumpsit, the former being an undertaking, made orally, by writing not under seal, or matter of record, to perform an act or to pay a sum of money to another; while an implied assumpsit was an undertaking presumed in law to have been made by a party from his conduct, although he had not made an express promise. The action of assumpsit was also divided into special assumpsit, or an action brought "on an express promise, and general assumpsit or an action brought on an implied contract. See Ann. Cas. 1913A, 471.
-- Ballentine's Law Dictionary