WordNet (r) 3.0 (2006):

weigh down
    v 1: be oppressive or disheartening to [syn: weigh down,
         weigh on] [ant: buoy up, lighten, lighten up]
    2: exert a force with a heavy weight; "The snow bore down on the
       roof" [syn: drag down, bear down, bear down on, press
       down on, weigh down]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "weigh down":
   ballast, bear hard upon, burden, cramp, cripple, cumber, embarrass,
   encumber, enmesh, ensnarl, entangle, entoil, entrammel, entrap,
   entwine, fetter, go hard with, go ill with, hamper, hamstring,
   handicap, hobble, impede, involve, lame, lead, lie on, lime, load,
   lumber, net, oppress, overburden, overload, press, press down,
   sadden, saddle with, sandbag, shackle, snarl, tangle, toil,
   trammel, try one, weigh heavy on, weigh on, weigh upon, weight,
   weight down