cud: the partly digested food a grazing animal brings back up from its stomach to chew a second time.
The cud is the wad of partly digested food that a grazing animal like a cow or sheep returns from its stomach to its mouth to chew again. This word names that food, and it appears in the Bible in only one setting: the food laws that sort land animals into those Israel could eat and those it could not.
The test there comes in two parts. An animal was clean, and so allowed as food, only if it both had a split hoof and chewed the cud; doing just one of the two was not enough. The camel, the rock badger, and the hare are listed as chewing the cud without a split hoof, and the pig as the opposite, split-hoofed but not chewing the cud. Each fails one half of the test and so counts as unclean.
The Hebrew wording is more vivid than the usual English "chews the cud." It says literally that the animal "brings up" the cud, picturing the food raised back into the mouth to be worked over again. This word for the food and the verb for bringing it up travel together as a set phrase everywhere the lists use them.