Cud is a portion of food that returns from a
ruminant's stomach to the mouth to be chewed for the second time. More accurately, it is a
bolus of semi-degraded food
regurgitated from the
reticulorumen of a
ruminant. Cud is produced during the physical digestive process of rumination. The idiomatic expression
chewing one's cud means
meditating or
pondering; similar expressions such as "he chewed that over for a bit", or "chew on
that!" likely have the same derivation.