When you ruminate, you chew something over, either literally or figuratively. Literal rumination may seem a little gross to humans, but to cows, chewing your cud (that's partially digested food brought up from the stomach for another chew) is just a natural part of life. Figurative ruminating is much more palatable to humans; that kind of deep, meditative thought is often deemed quite a worthy activity. The verb ruminate has described metaphorical chewing over since the early 1500s and actual chewing since later that same century. Our English word derives from and shares the meanings of the Latin ruminari, which in turn derives from rumen, the Latin name for the first stomach compartment of ruminant animals (that is, creatures like cows that chew their cud).