"to bring into being," early 15c., from Latin creatus, past participle of creare "to make, bring forth, produce, procreate, beget, cause," related to Ceres and to crescere "arise, be born, increase, grow," from PIE root *ker- (2) "to grow." De Vaan writes that the original meaning of creare "was 'to make grow', which can still be found in older texts ...." Related: Created; creating.
early 15c., "in, of, or pertaining to the mind; characteristic of the intellect," from Late Latin mentalis "of the mind," from Latin mens (genitive mentis) "mind," from PIE root *men- (1) "to think."
In Middle English, also "of the soul, spiritual." From 1520s as "done or performed in the mind." Meaning "crazy, deranged" is by 1927, probably from combinations such as mental patient (1859); mental hospital (1891). Mental health is attested by 1803; mental illness by 1819; mental retardation by 1904.
mid-15c., acten, "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").
The verb is original in Latin, but most of the modern verbal senses in English probably are from the noun. The general sense of "to do, perform, transact" is from c. 1600. Of things, "do something, exert energy or force," by 1751. In theater use from 1590s as "perform as an actor" (intransitive), 1610s as "represent by performance on the stage" (transitive). The general 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 (1640s). 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.
Old English be- (unstressed) or bi (stressed) "near, in, by, during, about," from Proto-Germanic *bi "around, about," in compounds often merely intensive (source also of Old Saxon and Old Frisian bi "by, near," Middle Dutch bie, Dutch bij, German bei "by, at, near," Gothic bi "about"), from PIE *bhi, reduced form of root *ambhi- "around."
As an adverb by c. 1300, "near, close at hand."
OED (2nd ed. print) has 38 distinct definitions of it as a preposition. Originally an adverbial particle of place, which sense survives in place names (Whitby, Grimsby, etc., also compare rudesby). Elliptical use for "secondary course" was in Old English (opposed to main, as in byway, also compare by-blow "illegitimate child," 1590s, Middle English loteby "a concubine," from obsolete lote "to lurk, lie hidden"). This also is the sense of the second by in the phrase by the by (1610s).
By the way literally means "along the way" (c. 1200), hence "in passing by," used figuratively to introduce a tangential observation ("incidentally") by 1540s. To swear by something or someone is in Old English, perhaps originally "in the presence of." Phrase by and by (early 14c.) originally meant "one by one," with by apparently denoting succession; modern sense of "before long" is from 1520s.
By and large "in all its length and breadth" (1660s) originally was nautical, "sailing to the wind and off it," hence "in one direction then another;" from nautical expression large wind, one that crosses the ship's line in a favorable direction.
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.