The first citation in the OED for its use in the adjectival form is Johnson, and it appears in his dictionary. He's simply given it its etymological meaning in Greek: from the verb keleuo 'to exhort, or order forward'. The rhetorical term used this metaphorically. Maybe NCOs have always used short, barked out commands with their troops. The PIE root *kleu- also gives us English loud.

Ceci n'est pas un seing.