1660s, from Latin ruminantem (nominative ruminans), present participle of ruminare "to chew the cud" (see ruminate). As an adjective from 1670s.
ruminant ru·mi·nant (rōō'mə-nənt)
n.
Any of various hoofed, even-toed, usually horned mammals of the suborder Ruminantia, such as cattle, sheep, goats, deer, and giraffes, characteristically having a stomach divided into four compartments and chewing a cud consisting of regurgitated, partially digested food.