c. 1300, "a gathering of persons, a group gathered for some purpose," from Old French as(s)emblee "assembly, gathering; union, marriage," noun use of fem. past participle of assembler "to assemble" (see assemble). Meaning "a gathering together" is recorded from early 15c.; that of "act of assembling parts or objects" is from 1914, as is assembly line.
Perhaps the most interesting department in the whole factory, to the visitor, is the final assembly. In this division, all the assembled units meet the assembly conveyor at the point where they are needed. At the start of the track a front axle unit, a rear axle unit and a frame unit are assembled. This assembly is then started in motion by means of a chain conveyor, and as it moves down the room at a constant speed of eight feet per minute, each man adds one part to the growing chassis or does one operation, which is assigned to him, so that when the chassis reaches the end of the line, it is ready to run on its own power. ["The Story of an Automobile Factory," in "Universal Book of Knowledge and Wonders," 1917]
School sense, "gathering of all students for a presentation" is from 1932. From mid-14c. as "a gathering for deliberation," hence it is the name of the lower house in state (earlier colonial) legislatures in America (1680s). In 17c.-18c. assemblies "dancing balls 'among polite persons of both sexes,' often paid for by subscription of the participants" were a prominent feature of social life.
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.