The Collaborative International Dictionary of English v.0.48:

Burdensome \Bur"den*some\, a.
   Grievous to be borne; causing uneasiness or fatigue;
   oppressive.
   [1913 Webster]

         The debt immense of endless gratitude
         So burdensome.                           --Milton.
   [1913 Webster]

   Syn: Heavy; weighty; cumbersome; onerous; grievous;
        oppressive; troublesome.
        [1913 Webster] -- Bur"den*some*ly, adv. --
        Bur"den*some*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

burdensome
    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:

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