"cloud of bees or other insects," Old English swearm "swarm, multitude," from Proto-Germanic *swarmaz (source also of Old Saxon, Middle Low German swarm, Danish sværm "a swarm," Swedish svärm, Middle Dutch swerm, Old High German swaram, German Schwarm "swarm;" Old Norse svarmr "tumult"), by Watkins, etc., derived from PIE imitative root *swer- "to buzz, whisper" (see susurration) on notion of humming sound, and thus probably originally of bees. But OED suggests possible connection with base of swerve and ground sense of "agitated, confused, or deflected motion." General sense "large, dense throng" is from early 15c.
"fanatical enthusiasm for a cause or person, blindly shared by the masses;" 1845, a German word in a specialized philosophical sense taken whole into English (in the Edinburgh Review, which adds "a word untranslatable, because the thing itself is un-English"), from German Schwärmerei "noisy, dissolute pleasures; religious fanaticism," from schwärmen "to swarm," figuratively "to be enthusiastic" (related to the noun Schwarm; see swarm (n.)).
The second element might be the German equivalent of -ery, but the sense is not clear. Perhaps the meaning in -rei is essentially diminutive and also denotes ridiculousness or contemptibility [a suggestion from D. Boileau, "Nature and Genius of the German Language," 1843, who cites among other examples Liebelei, "vulgar, insipid sweet-hearting," Ausländerei "too-frequent use of foreign words." Also compare later Schweinerei, "obnoxious behavior; a repulsive incident or thing," literally "piggishness"].
Used in German by Kant, Schelling, Hölderlin; used in late 19c. English by Carlyle, Ruskin, etc.
But we are in hard times, now, for all men's wits; for men who know the truth are like to go mad from isolation; and the fools are all going mad in "Schwärmerei,"—only that is much the pleasanter way. [Ruskin, "Fors Clavigera"]
In mid-20c. also "schoolgirl crush." Related: Schwärmerisch.
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.
"a whispering, a murmur," c. 1400, from Latin susurrationem (nominative susurratio), from past participle stem of susurrare "to hum, murmur," from susurrus "a murmur, whisper," a reduplication of the PIE imitative *swer- "to buzz, whisper" (source also of Sanskrit svarati "sounds, resounds," Greek syrinx "flute," Latin surdus "dull, mute," Old Church Slavonic svirati "to whistle," Lithuanian surma "pipe, shawm," German schwirren "to buzz," Old English swearm "a swarm").
It forms all or part of: abduce; abducent; abduct; abduction; adduce; aqueduct; circumduction; conduce; conducive; conduct; conductor; conduit; deduce; deduction; dock (n.1) "ship's berth;" doge; douche; ducal; ducat; Duce; duchess; duchy; duct; ductile; duke (n.); educate; education; induce; induction; introduce; introduction; misconduct; produce; production; reduce; reduction; seduce; seduction; subduce; subduction; taut; team (n.); teem (v.1) "abound, swarm, be prolific;" tie (n.); tow (v.); traduce; transducer; tug; zugzwang.
It is the hypothetical source of/evidence for its existence is provided by: Latin dux (genitive ducis) "leader, commander," in Late Latin "governor of a province," ducere "to lead;" Old English togian "to pull, drag," teonteon "to pull, drag;" German Zaum "bridle," ziehen "to draw, pull, drag;" Middle Welsh dygaf "I draw."