Source : Webster's Revised Unabridged Dictionary (1913)

Rancid \Ran"cid\, 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.

Source : WordNet®

rancid
     adj 1: used of decomposing oils or fats; "rancid butter"; "rancid
            bacon"
     2: smelling of fermentation or staleness [syn: {sour}]