Source : Webster's Revised Unabridged Dictionary (1913)

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.

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