A hooved animal that ferments food in their stomachs before digesting it. They then regurgitate it into their mouths and chew it again to break it down further. This is known as “chewing the cud.”
The category includes cattle, goats, sheep – most things that graze on grass, it seems.
Interestingly, this is related to “ruminate,” the concept of thinking very carefully and repetitively about something. Sources differ on which usage came first, but the central metaphor is to “re-chew” or “re-process” something – either by physically regurgitating it and chewing it again, or turning it around in your head over and over.