1630s, "a seal," Scottish borrowing of French cachet "seal affixed to a letter or document" (16c.), from Old French dialectal cacher "to press, crowd," from Latin coactare "constrain" (see cache). Meaning evolved 18c. (via French lettre de cachet "letter under seal of the king") to "(letter under) personal stamp (of the king)," thence to "symbol of prestige" (1840).
