actyoudaoicibaDictYouDict[act 词源字典]
act: [14] Act, action, active, actor all go back to Latin agere ‘do, perform’ (which is the source of a host of other English derivatives, from agent to prodigal). The past participle of this verb was āctus, from which we get act, partly through French acte, but in the main directly from Latin. The Latin agent noun, āctor, came into the language at about the same time, although at first it remained a rather uncommon word in English, with technical legal uses; it was not until the end of the 16th century that it came into its own in the theatre (player had hitherto been the usual term).

Other Latin derivatives of the past participial stem āct- were the noun āctiō, which entered English via Old French action, and the adjective āctīvus, which gave English active. See also ACTUAL.
=> action, active, agent, cogent, examine, prodigal[act etymology, act origin, 英语词源]
act (n.)youdaoicibaDictYouDict
late 14c., "a thing done," from Old French acte "(official) document," and directly from Latin actus "a doing, a driving, impulse; a part in a play, act," and actum "a thing done," originally a legal term, both from agere "to do, set in motion, drive, urge, chase, stir up," from PIE root *ag- "to drive, draw out or forth, move" (cognates: Greek agein "to lead, guide, drive, carry off," agon "assembly, contest in the games," agogos "leader;" Sanskrit ajati "drives," ajirah "moving, active;" Old Norse aka "to drive;" Middle Irish ag "battle").

Theatrical ("part of a play," 1510s) and legislative (early 15c.) senses of the word also were in Latin. Meaning "display of exaggerated behavior" is from 1928. In the act "in the process" is from 1590s, perhaps originally from the 16c. sense of the act as "sexual intercourse." Act of God "uncontrollable natural force" recorded by 1726.
An act of God is an accident which arises from a cause which operates without interference or aid from man (1 Pars. on Cont. 635); the loss arising wherefrom cannot be guarded against by the ordinary exertions of human skill and prudence so as to prevent its effect. [William Wait, "General Principles of the Law," Albany, 1879]
act (v.)youdaoicibaDictYouDict
mid-15c., "to act upon or adjudicate" a legal case; 1590s in the theatrical sense, from Latin actus, past participle of agere (see act (n.)). To act up "be unruly" is from 1903. To act out "behave anti-socially" (1974) is from psychiatric sense of "expressing one's unconscious impulses or desires." Related: Acted; acting.