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.
Old English heord "herd, flock," from Proto-Germanic *herdo- (cf. Old Norse hjorð, Old High German herta, German Herde, Gothic hairda "herd"), from PIE *kerdh- "a row, group, herd" (cf. Sanskrit śárdhah "herd, troop," Old Church Slavonic čreda "herd," Greek korthys "heap," Lithuanian kerdžius "shepherd"). Herd instinct in psychology is first recorded 1908.
Keep close watch or tight control over, as in Aunt Martha is always riding herd on her bridge club, making sure they follow the rules. This idiom alludes to the cowboy who rides around a herd of cattle to keep them together. [Late 1800s]