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]

WordNet (r) 3.0 (2006):

onerous
    adj 1: not easily borne; wearing; "the burdensome task of
           preparing the income tax return"; "my duties weren't
           onerous; I only had to greet the guests"; "a taxing
           schedule" [syn: burdensome, onerous, taxing]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "onerous":
   Herculean, annoying, arduous, arduously, backbreaking, besetting,
   bothersome, burdensome, burdensomely, crushing, cumbersome,
   cumbrous, demanding, difficult, difficultly, effortful,
   encumbering, exacting, exhausting, exigent, forced, grievous,
   grinding, grueling, hampering, hard, hard-earned, hard-fought,
   hardly, harsh, heavy, heavy-handed, hefty, impedimental, impeding,
   impeditive, incumbent, irksome, killing, labored, laborious,
   laboriously, lumpish, massive, onerously, operose, oppressive,
   overburdensome, painful, plaguey, ponderous, punishing, strained,
   strenuous, superincumbent, taxing, toilsome, toilsomely, tough,
   troublesome, trying, tyrannous, unruly, unwieldy, uphill,
   vexatious, wearing, wearisome, wearying, weighty