"cause (something) to happen," 1930, an image from trigger (n.). In recent use especially psychological, "to cause an intense and usually negative emotional reaction (in a person or animal)," by 1986. Related: Triggered; triggering.
mid-15c. as "a rejoinder, a reply" (a sense now archaic or obsolete); 1520s, "the return or bounding back of something after striking, act of flying back on collision with another body" in reference to a ball, from rebound (v.); rebounding in this sense is from late 14c.
In modern sports, from 1917 in ice hockey, 1920 in basketball. Transferred and figurative senses from 1560s; the meaning "period of reaction or renewed activity after disturbance" is from 1570s, hence "during a period of reaction after the end of a romantic or marital relationship" (1859).
1640s, "one who or that which checks or delays," agent noun from retard (v.). Scientific sense of "substance which slows down a reaction" by 1878. Specifically of braking mechanisms by 1937, originally on railroad cars.
also Mr. Chad, simple graffiti drawing of a head peering over a fence or wall, with the caption, "Wot, no ______?" (the U.S. version usually had "Kilroy was here"), 1945, British, of unknown origin, a reaction to war-time shortages and rationing.
"of or pertaining to a severe allergic reaction," 1905, with -ic + medical Latin noun anaphylaxis "exaggerated susceptibility," from Greek ana- "up" (see ana-) + phylaxis "protection," from phylax "guardian, watcher, protector," a word of unknown origin. Compare prophylactic. Anaphylactic shock is attested by 1916.