The Collaborative International Dictionary of English v.0.48:

Rancid \Ran"cid\ (r[a^]n"s[i^]d), a. [L. rancidus, fr. rancere
   to be rancid or rank.]
   Having a rank smell or taste, from chemical change or
   decomposition; musty; as, rancid oil or butter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rancid
    adj 1: (used of decomposing oils or fats) having a rank smell or
           taste usually due to a chemical change or decomposition;
           "rancid butter"; "rancid bacon"
    2: smelling of fermentation or staleness [syn: sour, rancid]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "rancid":
   awful, bad, bad-smelling, barfy, blown, brackish, cloying, corrupt,
   decayed, disagreeable, fecal, fetid, foul, frowsty, frowy, frowzy,
   fulsome, funky, fusty, gamy, graveolent, high, icky, ill-smelling,
   loathsome, maggoty, malodorous, mawkish, mephitic, miasmal,
   miasmic, mildewed, mildewy, moldy, musty, nasty, nauseant,
   nauseating, nauseous, nidorous, noisome, noxious, odious, odorous,
   off, offensive, olid, overripe, poisonous, putrid, rank, reasty,
   reasy, rebarbative, reechy, reeking, reeky, repulsive, ripe,
   rotten, sickening, smellful, smelling, smelly, sour, soured,
   spoiled, spoilt, stale, stenchy, stinking, strong, stuffy,
   sulfurous, tainted, turned, vile, vomity, weevily, yucky