Etymology
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react (v.)

1640s, "to exert, as a thing acted upon, an opposite action upon the agent," from re- "back" + act (v.). Related: Reacted; reacting (1610s). For sense development, see reaction. The verb meaning "to perform again, do a second time" (often written re-act and given full pronunciation of the prefix to distinguish it from react) is from 1650s, from the "again" sense in re-.

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overreact (v.)

also over-react, "respond with excessive force or emotion," by 1928, from over- + react (v.). Related: Overreacted; overreacting; overreaction.

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reactant (n.)

"a reacting thing," 1901, from react + -ant. As an adjective by 1911. Related: Reactance (1893).

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reactor (n.)

"one that reacts," 1835, agent noun in Latin form from react. By 1915 in electricity as "coil or other piece of equipment which provides reactance in a circuit;" the nuclear sense is attested from 1945.

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reactive (adj.)

1712, "repercussive, echoing," a sense now obsolete, from react + -ive. By 1822 as "caused by a reaction;" 1888 as "susceptible to (chemical) reaction." Related: Reactively; reactiveness; reactivity.

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*ag- 

Proto-Indo-European root meaning "to drive, draw out or forth, move."

It forms all or part of: act; action; active; actor; actual; actuary; actuate; agency; agenda; agent; agile; agitation; agony; ambagious; ambassador; ambiguous; anagogical; antagonize; apagoge; assay; Auriga; auto-da-fe; axiom; cache; castigate; coagulate; cogent; cogitation; counteract; demagogue; embassy; epact; essay; exact; exacta; examine; exigency; exiguous; fumigation; glucagon; hypnagogic; interact; intransigent; isagoge; litigate; litigation; mitigate; mystagogue; navigate; objurgate; pedagogue; plutogogue; prodigal; protagonist; purge; react; redact; retroactive; squat; strategy; synagogue; transact; transaction; variegate.

It is the hypothetical source of/evidence for its existence is provided by: Greek agein "to lead, guide, drive, carry off," agon "assembly, contest in the games," agōgos "leader," axios "worth, worthy, weighing as much;" Sanskrit ajati "drives," ajirah "moving, active;" Latin actus "a doing; a driving, impulse, a setting in motion; a part in a play;" agere "to set in motion, drive, drive forward," hence "to do, perform," agilis "nimble, quick;" Old Norse aka "to drive;" Middle Irish ag "battle."

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catharsis (n.)

1770, "a bodily purging" (especially of the bowels), from Latinized form of Greek katharsis "purging, cleansing," from stem of kathairein "to purify, purge," from katharos "pure, clear of dirt, clean, spotless; open, free; clear of shame or guilt; purified" (with most of the extended senses now found in Modern English clear, clean, pure), which is of unknown origin.

Originally medical in English; of emotions, "a purging through vicarious experience," from 1872; psychotherapy sense first recorded 1909, in Brill's translation of Freud's "Selected Papers on Hysteria."

The German abreagiren has no exact English equivalent. It will therefore be rendered throughout the text by "ab-react," the literal meaning is to react away from or to react off. It has different shades of meaning, from defense reaction to emotional catharsis, which can be discerned from the context. [footnote, pp. 5-6]
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trigger (n.)

"device by means of which a catch or spring is released and a mechanism set in action," 1650s, earlier tricker (1620s), from Dutch trekker "trigger," from trekken "to pull" (see trek). Tricker was the usual form in English until c. 1750. Trigger-finger "forefinger as used to pull the trigger of a gun" is attested by 1814. Trigger-happy "ready to shoot (or otherwise react violently) on the slightest provocation" is attested from 1942.

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bop (n.)
1948, shortening of bebop or rebop. The musical movement had its own lingo, which was in vogue in U.S. early 1950s. "Life" magazine [Sept. 29, 1952] listed examples of bop talk: crazy "new, wonderful, wildly exciting;" gone (adj.) "the tops--superlative of crazy;" cool (adj.) "tasty, pretty;" goof "to blow a wrong note or make a mistake;" hipster "modern version of hepcat;" dig "to understand, appreciate the subtleties of;" stoned "drunk, captivated, ecstatic, sent out of this world;" flip (v.) "to react enthusiastically."
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reaction (n.)

"action in resistance or response to another action or power," 1640s, from re- "back, again, anew" + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), a noun of action formed in Late Latin from the past-participle stem of Latin reagere "react," from re- "back" + agere "to do, perform."

Originally a word in physics and dynamics. In chemistry, "mutual or reciprocal action of chemical agents upon each other," by 1836. The general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.

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