"short, pithy, instructive saying," 1550s, from Greek apophthegma "terse, pointed saying," literally "something clearly spoken," from apophthengesthai "to speak one's opinion plainly," from apo "from" (see apo-) + phthengesthai "to utter" (see diphthong). See aphorism for nuances of usage. Spelling apophthegm, restored by Johnson, is "now more frequent in England," according to OED (1989). Related: Apothegmatic.
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Harper, D. (n.d.). Etymology of apophthegm. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/apophthegm