1630s, Scottish borrowing of Fr. cachet
"seal affixed to a letter or document" (16c.), from O.Fr. dialectal cacher
"to press, crowd," from L. coactare
"constrain" (see cache
). Meaning evolving through "(letter under) personal stamp (of the king)" to "prestige." Cf. Fr. lettre de cachet
"letter under seal of the king."