Etymology
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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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*ger- 

Proto-Indo-European root meaning "to gather." 

It forms all or part of: aggregate; aggregation; agora; agoraphobia; allegory; category; congregate; cram; egregious; gregarious; panegyric; paregoric; segregate.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit gramah "heap, troop;" Greek ageirein "to assemble," agora "assembly;" Latin grex "flock, herd," gremium "bosom, lap;" Old Church Slavonic grusti "handful," gramota "heap;" Lithuanian gurgulys "chaos, confusion," gurguolė "crowd, mass;" Old English crammian "press something into something else."

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

"a colony of rooks, a place where rooks congregate to breed," 1725, from rook (n.1) + -ery.

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roll (v.)

early 14c., rollen, "turn over and over, move by rotating" (intransitive); late 14c. in the transitive sense of "move (something) by turning it over and over;" from Old French roeller "roll, wheel round" (Modern French rouler), from Medieval Latin rotulare, from Latin rotula, diminutive of rota "wheel" (see rotary). Related: Rolled; rolling.

From c. 1400 as "wrap or cover by rolling or enclosing" in something, also "wrap round and round an axis;" early 15c. as "press or level with a roller." From 1510s as "to move or travel on wheels or by means of rolling." Of sounds (such as thunder) somehow suggestive of a rolling ball, 1590s; of a drum from 1680s.

Of spoken sounds, "to utter with vibrations of the tongue," by 1846. Of eyes, from late 14c. (rolle his eyne), originally suggestive of ferocity or madness. Of a movie camera, "to start filming," from 1938. Sense of "rob a stuporous drunk" is by 1873, from the action required to get to his pockets. To roll up "gather, congregate" is from 1861, originally Australian. To roll with the punches is a metaphor from boxing (1940). To roll them bones was old slang for "play at dice" (1929). Heads will roll is a Hitlerism:

If our movement is victorious there will be a revolutionary tribunal which will punish the crimes of November 1918. Then decapitated heads will roll in the sand. [1930]
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