late 14c., perspectif, "the science of optics," from Old French perspective and directly from Medieval Latin perspectiva ars "science of optics," from fem. of perspectivus "of sight, optical" from Latin perspectus "clearly perceived," past participle of perspicere "inspect, look through, look closely at," from per "through" (from PIE root *per- (1) "forward," hence "through") + specere "look at" (from PIE root *spek- "to observe"). The English word is also attested from early 15c. as an adjective, "pertaining to the science of optics."
The sense of "the art of drawing solid objects on a flat surface so as to give appearance of distance or depth" is attested by 1590s, probably by influence of Italian prospettiva, an artists' term. The meaning "proper or just proportion, appropriate relation in the mind of the parts of a subject to one another" is recorded by c. 1600, hence the figurative meaning "mental outlook over time" (1762).
early 14c., transitive ("collect into one place") and intransitive ("meet or come together"), from Old French assembler "come together, join, unite; gather" (11c.), from Latin assimulare "to make like, liken, compare; copy, imitate; feign, pretend," later "to gather together," from assimilated form of ad "to" (see ad-) + simulare "to make like," from stem of similis "like, resembling, of the same kind" (see similar).
In Middle English and in Old French it also was a euphemism for "to couple sexually." The meaning "put parts together" in manufacturing is from 1852. Related: Assembled; assembling. Assemble together is redundant.
late 15c., "visit or associate with," from Old French frequenter "attend frequently; assemble, gather together," from Latin frequentare "visit regularly; do frequently, repeat; assemble in throngs," from frequentem (see frequent (adj.)). Meaning "visit often" is from 1550s. Related: Frequented; frequenter; frequenting.
1935, trade name in Britain for what in the U.S. is called Plexiglas or Lucite, irregularly formed from Latin perspect-, past participle stem of perspicere "look through, look closely at" (see perspective).
1560s, "to assemble," from troop (n.). Meaning "to march" is recorded from 1590s; that of "to go in great numbers, to flock" is from c. 1600. Related: Trooped; trooping.