mid-15c., "to act upon or adjudicate" a legal case, from Latin actus, past participle of agere "to set in motion, drive, drive forward," hence "to do, perform," also "act on stage, play the part of; plead a cause at law" (from PIE root *ag- "to drive, draw out or forth, move").
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 by 1900 (in reference to a horse). Earlier it meant "acting in accordance with" a duty, expectation, or belief (1645). 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.
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 set in motion, drive, drive forward," hence "to do, perform," figuratively "incite to action; keep in movement, stir up," a verb with a broad range of meaning in Latin, including "act on stage, play the part of; plead a cause at law; chase; carry off, steal;" from PIE root *ag- "to drive, draw out or forth, move."
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.
signed into law Oct. 26, 2001; a contrived acronym for the Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001.