Is it ensure, insure, or assure?
1590s (implied in tersely), "clean-cut, burnished, neat," from French ters "clean," from Latin tersus "wiped off, clean, neat," from past participle of tergere "to rub, polish, wipe." 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.