Old English heord "herd, flock," from Proto-Germanic *herdo- (cognates: Old Norse hjorð, Old High German herta, German Herde, Gothic hairda "herd"), from PIE *kerdh- "a row, group, herd" (cognates: Sanskrit śárdhah "herd, troop," Old Church Slavonic čreda "herd," Greek korthys "heap," Lithuanian kerdžius "shepherd"). Herd instinct in psychology is first recorded 1908.

mid-13c., “to watch over or herd (livestock);” of animals, “to gather in a herd, to form a flock,” late 14c., from herd, n.. Related: Herded; herding.