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):

burdensomeness
    n 1: unwelcome burdensome difficulty [syn: burdensomeness,
         heaviness, onerousness, oppressiveness]