Old English flocc "a group of persons, company, troop," related to Old Norse flokkr "crowd, troop, band," Middle Low German vlocke "crowd, flock (of sheep);" of unknown origin, not found in other Germanic languages; perhaps related to folc "people," but the metathesis would have been unusual for Old English.
In Old English of humans only; extended c. 1200 to "a number of animals of one kind moving or feeding together;" of domestic animals c. 1300. The special reference to birds is recent (19c.). Transferred to bodies of Christians, in relation to Christ or their pastor, from mid-14c.
"tuft of wool," mid-13c., also found in continental Germanic and Scandinavian, all probably from Old French floc, from Latin floccus "tuft of wool, lock of hair," a word of unknown origin.
c. 1300 "gather, congregate" (intransitive), from flock (n.1). Related: Flocked; flocking.