Etymology
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flock (n.2)
"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.
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flock (v.)
c. 1300 "gather, congregate" (intransitive), from flock (n.1). Related: Flocked; flocking.
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flock (n.1)
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.
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aggregate (adj.)

c. 1400, from Latin aggregatus "associated, united," past participle of aggregare "add to (a flock), lead to a flock, bring together (in a flock)," figuratively "attach, join, include; collect, bring together," from ad "to" (see ad-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather").

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gregarious (adj.)

1660s, "disposed to live 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," from PIE *gre-g-, reduplicated form of root *ger- "to gather." Of persons, "sociable," first recorded 1789. Related: Gregariously; gregariousness.

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congregate (v.)

mid-15c. (implied in congregated), "accumulate," originally of fluids in the body, from Latin congregatus, past participle of congregare "to herd together, collect in a flock, swarm; assemble," from assimilated form of com "together" (see con-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather").

Of persons, "collect or bring together in an assembly," 1510s; intransitive sense "come together, assemble or meet in large numbers," 1530s. Related: Congregating.

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aggregate (v.)

c. 1400, "bring together in a sum or mass," from Latin aggregatus, past participle of aggregare "attach, join, include; collect, bring together," literally "bring together in a flock," from assimilated form of ad "to" (see ad-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather"). The intransitive meaning "come together in a sum or mass" is from 1855. Related: Aggregated; aggregating.

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black sheep (n.)

by 1792, in the figurative sense of "person of bad character; member of some group guilty of offensive conduct that does little credit to the flock, family, or community to which he belongs," supposedly because a real black sheep (there was proverbially one in every flock) had wool that could not be dyed and thus was of less worth. But one black sheep in a flock was considered good luck by shepherds in Sussex, Somerset, Kent, Derbyshire. First known publication of Baa Baa Black Sheep nursery rhyme is in "Tommy Thumb's Pretty Song Book" (c. 1744).

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aggregation (n.)

early 15c., aggregacioun, originally in medicine (Chauliac), "formation of a pustule," from Medieval Latin aggregationem (nominative aggregatio), noun of action from past-participle stem of Latin aggregare "collect, bring together," from ad "to" (see ad-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather"). From 1540s as "a combined whole;" by 1560s as "act of collecting in an unorganized mass."

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sord (n.)
"flock of mallards," 15c., perhaps from sord (v.) "to take flight," from Old French sordre "arise, stand up," from Latin surgere "to rise" (see surge (n.)).
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