C16: from Latin rūmināre to chew the cud, from rumen
Word Origin and History for ruminate
v.
1530s, "to turn over in the mind," also "to chew cud" (1540s), from Latin ruminatus, past participle of ruminare "to chew the cud; turn over in the mind," from rumen (genitive ruminis) "gullet," of uncertain origin. Related: Ruminated; ruminating.