one-act (adj.)

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" (from PIE root *ag- "to drive, draw out or forth, move").

The verb agere had a broad range of meaning in Latin, including "act on stage, play the part of; plead a cause at law; chase; carry off, steal." The theatrical ("part of a play," 1510s) and legislative (early 15c.) senses of the noun also were in Latin.

The meaning "one of a series of performances in a variety show" is from 1890. The 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 a late 16c. sense of the act as "sexual intercourse." Act of God "uncontrollable natural force" is 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, perhaps euphemistic.

"being but a single unit or individual; being a single person, thing, etc. of the class mentioned;" as a pronoun, "a single person or thing, an individual, somebody;" as a noun, "the first or lowest of the cardinal numerals; single in kind, the same; the first whole number, consisting of a single unit; unity; the symbol representing one or unity;" c. 1200, from Old English an (adjective, pronoun, noun) "one," from Proto-Germanic *ainaz (source also of Old Norse einn, Danish een, Old Frisian an, Dutch een, German ein, Gothic ains), from PIE root *oi-no- "one, unique."

Originally pronounced as it still is in only, atone, alone, and in dialectal good 'un, young 'un, etc.; the now-standard pronunciation "wun" began c. 14c. in southwest and west England (Tyndale, a Gloucester man, spells it won in his Bible translation), and it began to be general 18c. Its use as indefinite pronoun was influenced by unrelated French on and Latin homo.

Before the name of a person, indicating "hitherto unknown" or not known to the speaker.

One and only "sweetheart" is from 1906. Slang one-arm bandit for a type of slot machine is recorded by 1938. One-night stand is 1880 in performance sense; 1963 in sexual sense. One of the boys "ordinary amiable fellow" is from 1893. One-track mind "mind capable of only one line of thought or action" is by 1915. Drinking expression one for the road is from 1950 (as a song title). One-man band is by 1909 in a literal sense, 1914 figurative. One of those things "unpredictable occurrence" is from 1934 (Cole Porter's song is from 1935).

The conscience clause is one of the weaknesses of the Bill. It is one of those things which tend to create the bitterness. The conscience clause is one of those things which are inseparable from a Bill like this. It is one of those things which divides the sheep from the goats—members can pick them out for themselves—in the playground, in the school. ["Religious Exercises in School Bills," New Zealand Parliamentary Debates, Aug. 13, 1926]