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.
early 15c., (intransitive) "to come together, meet in the same place," usually for some public purpose, from Old French convenir "to come together; to suit, agree," from Latin convenire "to come together, meet together, assemble; unite, join, combine; agree with, accord; be suitable or proper (to)," from assimilated form of com "with, together" (see con-) + venire "to come," from a suffixed form of PIE root *gwa- "to go, come."
Transitive sense of "call together, cause to assemble" is from 1590s. Related: Convened; convener; convening.
early 15c., "collide, clash in hostility," from Latin concurrere "to run together, assemble hurriedly; clash, fight," in transferred use, "to happen at the same time," from assimilated form of com "together" (see con-) + currere "to run" (from PIE root *kers- "to run"). Sense of "to coincide, happen at the same time" is 1590s; that of "to agree in opinion" is 1580s in English.
"one who celebrates" in any sense, 1731, from French célébrant "officiating clergyman" (in celebrating the eucharist) or directly from Latin celebrantem (nominative celebrans), present participle of celebrare "assemble together; sing the praises of; practice often" (see celebrate).
early 14c., moustren, "to display, reveal, to show or demonstrate" (senses now obsolete), also "to appear, be present," from Old French mostrer "appear, show, reveal," also in a military sense (10c., Modern French montrer), from Latin monstrare "to show," from monstrum "omen, sign" (see monster).
The transitive meaning "to collect, assemble, bring together in a group or body," especially for military service or inspection, is from early 15c. The intransitive sense of "assemble, meet in one place," of military forces, is from mid-15c. The figurative use "summon, gather up" (of qualities, etc.) is from 1580s.
To muster in (transitive) "receive as recruits" is by 1837; to muster out "gather to be discharged from military service" is by 1834, American English. To muster up in the figurative and transferred sense of "gather, summon, marshal" is from 1620s. Related: Mustered; mustering.
1751 in botany, in reference to a class of flowers having 20 or more stamens; 1809 of human relationships (implied in polyandrian), from poly- "many" andr-, stem of aner "man, husband" (from PIE root *ner- (2) "man"), which is used in botany to mean "stamen, having stamens," + -ia "condition of." Late Greek polyandria meant "populousness," a polyandrion was "place where many assemble." Related: Polyandric.