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]

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

ONEROUS CAUSE, civil law., A valuable consideration.