WordNet (r) 3.0 (2006):

frowsty
    adj 1: stale and unclean smelling [syn: fusty, musty,
           frowsty]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "frowsty":
   bad, bad-smelling, blasted, blighted, blown, despoiled, fecal,
   fetid, flyblown, foul, frowsy, frowy, frowzy, fulsome, funky,
   fusty, gamy, graveolent, high, ill-smelling, maggoty, malodorous,
   mephitic, miasmal, miasmic, mildewed, mildewy, moldering, moldy,
   moth-eaten, musty, nidorous, noisome, noxious, odorous, offensive,
   olid, putrid, rancid, rank, ravaged, reasty, reasy, reechy,
   reeking, reeky, repulsive, rotten, smellful, smelling, smelly,
   smutted, smutty, stenchy, stinking, strong, stuffy, sulfurous,
   vile, weevily, worm-eaten, wormy