1350-1400;Middle Englishact(e) (< Middle French) < Latinācta, plural of āctum something done, noun use of past participle of agere to do (āg- past participle stem + -tum neuter past participle suffix); and directly < Latināctus a doing (āg- + -tus suffix of v. action)
late 14c., from O.Fr. acte, from L. actus "a doing" and actum "a thing done," both from agere "to do, set in motion, drive, urge, chase, stir up," from PIE root *ag- "to drive, draw out or forth, move" (cf. Gk. agein "to lead, guide, drive, carry off," agon "assembly, contest in the games," agogos "leader;" Skt. ajati "drives," ajirah "moving, active;" O.N. aka "to drive;" M.Ir. ag "battle"). Theatrical (1510s) and legislative (mid-15c.) senses of the word also were in Latin. The verb is first attested late 15c.; in the theatrical performance sense it is from 1590s. In the act "in the process" is from 1590s, originally from the 16c. sense of the act as "sexual intercourse." Act of God "uncontrollable natural force" first recorded 1882. To act out "behave anti-socially" (1974) is from psychiatric sense of "expressing one's unconscious impulses or desires."