1660s, "living in flocks" (of animals), from Latin gregarius "pertaining to a flock; of the herd, of the common sort, common," from grex (genitive gregis) "flock, herd," reduplication of PIE root *ger- "to gather together, assemble" (cognates: Sanskrit gramah "heap, troop;" Greek ageirein "to assemble," agora "assembly;" Latin gremium "bosom, lap;" Old Church Slavonic grusti "handful," gramota "heap;" Lithuanian gurgulys "chaos, confusion," gurguole "crowd, mass"). Sense of "sociable" first recorded 1789. Related: Gregariously; gregariousness.