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.

late 14c., "an overseer, guardian, steward," from Latin actor "an agent or doer; a driver (of sheep, etc.)," in law, "accuser, plaintiff," also "theatrical player, orator," from past-participle stem 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"). In English from mid-15c. as "a doer, maker," also "a plaintiff at law." The sense of "one who performs in plays" is by 1580s, originally applied to both men and women. Related: Actorish; actorly; actory.

late 14c., "power, efficacy," from Medieval Latin actualitatem (nominative actualitas), from Late Latin actualis "pertaining to action," from Latin actus "a doing" (from PIE root *ag- "to drive, draw out or forth, move"). A Latin loan-translation of Greek energeia "activity, action, operation" (see energy). Meaning "state of being real" is from 1670s (actualities "existing conditions" is from 1660s).

Mod. use of actuality in the sense of realism, contact with the contemporary, is due to Fr. actualité, from actuel, which does not mean actual, real, but now existing, up to date. [Weekley]

late 14c., originally of fevers and diseases, "coming quickly to a crisis" (opposed to chronic), from Latin acutus "sharp, pointed," figuratively "shrill, penetrating; intelligent, cunning," past participle of acuere "to sharpen" (literal and figurative), from PIE root *ak- "be sharp, rise (out) to a point, pierce."

It was also used of humors (early 15c.). The meaning "ending in a sharp point" is from 1560s; the sense of "sharp or penetrating in intellect" is from 1580s. Of feelings, pains, etc., "intense," 1727. As a noun, early 15c. of fevers; c. 1600 as "an acute accent." Related: Acutely; acuteness.

late 14c., "by day; on or in the day or time," with adverbial genitive -s from earlier aday (mid-13c.), prepositional phrase used as an adverb, from a- (1) "on, on each" + day (n.). The genitive ending now is regarded as an accusative plural.

late 14c., "hard, unbreakable," from adamant (n.). The figurative sense of "unshakeable" (in belief, etc.) is by 1670s. Related: Adamantly; adamance.

late 14c., "to join or unite (something to something else)," from Latin addere "add to, join, attach, place upon," literal and figurative, from ad "to" (see ad-) + -dere, combining form meaning "to put, place," from dare "to give" (from PIE root *do- "to give").

The intransitive meaning "to do sums, do addition" also is from late 14c. Related: Added; adding. To add up is from 1754; in the figurative meaning "make sense," by 1942. Adding machine "machine to cast up large sums" is from 1822.

late 14c., "action of adding numbers;" c. 1400, "that which is added," from Old French adition "increase, augmentation" (13c.), from Latin additionem (nominative additio) "an adding to, addition," noun of action from past-participle stem of addere "add to, join, attach" (see add). Phrase in addition to "also" is from 1680s.

late 14c., "sticking, clinging to, adhesive," from Old French adherent or directly from Latin adhaerentem (nominative adhaerens), present participle of adhaerere "stick to," from ad "to" (see ad-) + haerere "to stick" (see hesitation).

late 14c., adewe, from Old French a Dieu, a Deu, shortened from phrases such as a dieu (vous) commant "I commend (you) to God," from a "to" (see ad-) + dieu "God," from Latin deum, accusative of deus "god" (from PIE *deiwos "god" (from root *dyeu- "to shine").

Originally it was said to the party left (farewell was to the party setting forth), but in English it came to be used as a general parting salutation. As a noun, "expression of kind wishes upon departure," late 14c. Compare the native parting salutation good-bye, a contraction of God be with ye.