What is Onerous?

Legal Definition
A contract, lease, share or other right is said to be "onerous" when the obligations attaching to it counter-balance or exceed the advantage to be derived from it, either absolutely or with reference to the particular possessor. Sweet. As used in the civil law and in the systems derived from it, (French, Scotch, Spanish, Mexican,) the term also means based upon, supported by, or relating to a good and valuable consideration, i. e., one which imposes a burden or charge in return for the benefit cohf erred.
-- Black's Law Dictionary
Legal Definition
Burdensome; not without consideration.
-- Ballentine's Law Dictionary