The definition of rancid is food that tastes or smells gross because of old or spoiled oil or fat.(adjective)
Milk that has spoiled is an example of something that would be described as rancid.
See rancid in Webster's New World College Dictionary
Origin: L rancidus < rancere, to be rank
See rancid in American Heritage Dictionary 4
Origin: Latin rancidus
Origin: , from rancēre, to stink, be rotten.
Learn more about rancid