Onerous cause

from The Collaborative International Dictionary of English v.0.48

Onerous \On"er*ous\, a. [L. onerosus, fr. onus, oneris, a load,
   burden: cf. F. on['e]reux.]
   Burdensome; oppressive. "Too onerous a solicitude." --I.
   Taylor.
   [1913 Webster]

   {Onerous cause} (Scots Law), a good and legal consideration;
      -- opposed to {gratuitous}.
      [1913 Webster]

from Bouvier's Law Dictionary, Revised 6th Ed (1856)

ONEROUS CAUSE, civil law., A valuable consideration.