1590s (implied in tersely), "clean-cut, burnished, neat," from French ters "clean," and directly from Latin tersus "wiped off, clean, neat," from past participle of tergere "to rub, polish, wipe," which is of uncertain origin. Sense of "concise or pithy in style or language" is from 1777, which led to a general sense of "neatly concise." The pejorative meaning "brusque" is a fairly recent development. Related: Terseness.
"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. The spelling apophthegm, restored by Johnson, is "now more frequent in England," according to OED (1989). Related: Apothegmatic.