Etymology
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pang (n.)

1520s, "sudden paroxysm of physical pain, acute painful spasm," a word of unknown origin, not found in Middle English. Perhaps it is related to prong (prongys of deth is recorded from mid-15c.). Reference to mental or emotional pain is from 1560s. As a verb, "cause or suffer a pang or pangs," c. 1500. Related: Pangs.

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throe (n.)

c. 1200, throwe "pain, pang of childbirth, agony of death," of uncertain origin, possibly from Old English þrawan "twist, turn, writhe" (see throw (v.)), or altered from Old English þrea (genitive þrawe) "affliction, pang, evil; threat, persecution" (related to þrowian "to suffer"), from Proto-Germanic *thrawo (source also of Middle High German dro "threat," German drohen "to threaten"). Modern spelling first recorded 1610s. Related: Throes.

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stimulus (n.)

plural stimuli, 1680s, originally as a medical term, "something that goads a lazy organ" (often the male member), from a modern use of Latin stimulus "a goad, a pointed stick," figuratively "a sting, a pang; incitement, spur," from PIE *sti- "point, prick, pierce" (see stick (v.)). General sense of "something that excites or arouses the mind or spirit" is from 1791. Psychological sense is first recorded 1894.

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scruple (n.)

"moral misgiving, pang of conscience," late 14c., scrupul, from Old French scrupule (14c.), from Latin scrupulus "uneasiness, anxiety, pricking of conscience," also, literally, "small sharp stone," a diminutive of scrupus "sharp stone or pebble," used figuratively by Cicero for a cause of uneasiness or anxiety, a word of unknown etymology.

Probably the notion in the image is of a pebble in one's shoe. The word in the classical Latin sense of "smallest unit of weight or measurement" also is attested in English from late 14c., and was given various extensions: "one minute of arc, one minute of an hour," etc. The Latin words commonly are regarded as identical, with sense development of the latter from "small pebble" to "small weight."

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