act (n.) Look up act at
late 14c., "a thing done," from Latin actus "a doing; a driving, impulse, a setting in motion; a part in a play," and actum "a thing done" (originally a legal term), both from agere "to do, perform, transact," with a very broad range of meaning: "set in motion, put in motion; drive, drive away, chase; carry off, steal;" figuratively "lead, urge, impell, incite, stir up."

This is from PIE root *ag- (1) "to drive, draw out or forth, move" (source also of 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 "one of a series of performances in a variety show" is from 1890. Meaning "display of exaggerated behavior" is from 1928, extended from the theatrical sense. In the act "in the process" is from 1590s, perhaps originally from late 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]
To get into the act "participate" is from 1947; to get (one's) act together "organize one's (disorderly) life" is by 1976.
act (v.) Look up act at
mid-15c., "to act upon or adjudicate" a legal case, from Latin actus, past participle of agere "to do, set in motion" (see act (n.)). Most of the modern senses in English probably are from the noun. General sense of "to do, perform, transact" is from c. 1600. OF things, "do something, exert energy or force," by 1751. In the theater from 1590s as "perform as an actor" (intransitive), 1610s as "represent by performance on the stage" (transitive). Meaning "perform specific duties or functions," often on a temporary basis, is by 1804.

To act on "exert influence on" is from 1810. 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" (acting out is from 1945). Related: Acted; acting.