Online Etymology Dictionary

flock (n.): O.E. flocc "a group of persons," related to O.N. flokkr "crowd, troop, band," M.L.G. vlocke "crowd, flock;" not found in other Gmc. languages, perhaps related to folc "people," but the metathesis would have been unusual for O.E. Extended c.1200 to "a number of animals of one kind moving or feeding together;" of domestic animals c.1300. Transferred to bodies of Christians, in relation to Christ or their local pastor, from c.1340. The verb meaning "to gather, congregate" is from c.1300.
segregate: 1542, from L. segregatus, pp. of segregare "separate from the flock, isolate, divide," from *se gregare, from se "apart from" (see secret) + grege, ablative of grex "herd, flock." Originally often with ref. to the religious notion of separating the flock of the godly from sinners. Segregation (1555) is from L.L. segregatio, from L. segregatus; in the specific U.S. racial sense it is attested from 1903; segregationist is from the 1920s.
egregious: c.1534, from L. egregius, from the phrase ex grege "rising above the flock," from ex "out of" + grege, abl. of grex "herd, flock." Disapproving sense, now predominant, arose 16c., originally ironic and is not in the L. word, which etymologically means simply "exceptional."
aggregate (adj.): c.1400, from L. aggregatus "associated," lit. "united in a flock," pp. of aggregare "add to," from ad- "to" + gregare "herd," so "to lead to a flock" (see gregarious).
congregate: c.1400, from L. congregatus, pp. of congregare "to herd together," from com- "together" + gregare "to collect into a flock, gather," from grex (gen. gregis) "a flock."
troop (n.): 1545, "body of soldiers," from M.Fr. troupe, from O.Fr. trope "band of people, company, troop" (13c.), probably from Frank. *throp "assembly, gathering of people" (cf. O.E. ðorp, O.N. thorp "village," see thorp). OED derives the O.Fr. word from L. troppus "flock," which is of unknown origin but may be from the Gmc. source. The verb is attested from 1565, "to assemble;" meaning "to march" is recorded from 1592; that of "to go in great numbers, to flock" is from 1610. Trooper "soldier in the cavalry" is first attested 1640; extended to "mounted policeman" (1858, in Australian) then to "state policeman" (U.S.) by 1911.
bellwether: c.1440, from bell + wether; the lead sheep (on whose neck a bell was hung) of a domesticated flock; used earlier in the fig. sense of "chief, leader" (c.1430).
black sheep: figurative use is supposedly because a real black sheep had wool that could not be dyed and was thus worthless. But one black sheep in a flock was considered good luck by shepherds in Sussex, Somerset, Kent, Derbyshire. Baa Baa Black Sheep nursery rhyme's first known publication is in "Tommy Thumb's Pretty Song Book" (c.1744).
burlesque: 1667, "derisive imitation, grotesque parody," from Fr. burlesque, from It. burlesco, from burla "joke, fun, mockery," possibly ult. from L.L. burra "trifle, nonsense," lit. "flock of wool." Modern sense of "variety show featuring striptease" is Amer.Eng., 1870. Originally (1857) "the sketches at the end of minstrel shows."
gregarious: 1668, "living in flocks" (of animals), from L. gregarius, from grex (gen. gregis) "flock, herd," reduplication of PIE base *gere- "to gather together, assemble" (cf. Gk. ageirein "to assemble," agora "assembly;" O.C.S. grusti "handful;" Lith. gurgulys "chaos, confusion, gurguole "crowd, mass"). Sense of "sociable" first recorded 1789.
peculiar: c.1460, from L. peculiaris "of one's own (property)," from peculium "private property," lit. "property in cattle" (in ancient times the most important form of property), from pecu "cattle, flock," related to pecus "cattle" (see pecuniary). Meaning of "unusual" is first attested 1608; peculiarity "special characteristic" is from 1646; noun meaning "an oddity" is 1777.
frock: 1350, from O.Fr. froc "a monk's habit" (12c.), perhaps from Frank. *hroc (cf. O.H.G. hroc "mantle, coat;" O.N. rokkr, O.E. rocc, O.Fris. rokk, Ger. Rock "coat"), from PIE base *rug- "to spin." Another theory traces it to M.L. floccus, from L. floccus "flock of wool." Non-religious use is 1538.
pecuniary: 1502, from L. pecuniarius "pertaining to money," from pecunia "money, property, wealth," from pecu "cattle, flock," from PIE base *peku- (cf. Skt. pasu- "cattle," Goth. faihu "money, fortune," O.E. feoh "cattle, money"). Livestock was the measure of wealth in the ancient world. For a related sense development in O.E., see fee. Cf. also Welsh tlws "jewel," cognate with Ir. tlus "cattle," connected via notion of "valuable thing."
ewe: O.E. eowu, fem. of eow "sheep," from P.Gmc. *awi, gen. *awjoz (cf. M.Du. ooge, O.H.G. ouwi, Goth. aweþi "flock of sheep"), from PIE *owi- (cf. Skt. avih, Gk. ois, Lith. avis "sheep," O.C.S. ovica "ewe").