What is Legal Maxim?

Legal Definition
A legal maxim is an established principle or proposition. The Latin term, apparently a variant on maxima, is not to be found in Roman law with any meaning exactly analogous to that of a legal maxim in the Medieval or modern sense of the word, but the treatises of many of the Roman jurists on Regular definitiones, and Sententiae juris are, in some measure, collections of maxims. Most of the Latin maxims developed in the Medieval era in European countries that used Latin as their language for law and courts.

The attitude of early English commentators towards the maximal of the law was one of unmingled adulation. In Thomas Hobbes, Doctor and Student (p. 26), they are described as of the same strength and effect in the law as statutes. Not only, observes Francis Bacon in the Preface to his Collection of Maxims, will the use of maxims be in deciding doubt and helping soundness of judgment, but, further, in gracing argument, in correcting unprofitable subtlety, and reducing the same to a more sound and substantial sense of law, in reclaiming vulgar errors, and, generally, in the amendment in some measure of the very nature and complexion of the whole law.

A similar note was sounded in Scotland; and it has been well observed that a glance at the pages of Morrisons Dictionary or at other early reports will show how frequently in the older Scots law questions respecting the rights, remedies and liabilities of individuals were determined by an immediate reference to legal maxims.

In later times, less value has been attached to the maxims of the law, as the development of civilization and the increasing complexity of business relations have shown the necessity of qualifying the propositions which they enunciate. But both historically and practically, they must always possess interest and value.
-- Wikipedia