Yours, Etc.: Origins and Uses of 8 Sign-Offs
early 15c., "readiness," from Latin promptus (see prompt (v.)). Meaning "hint, act of prompting" is from 1590s. Computer sense attested by 1977.
early 15c., from Old French prompt and directly from Latin promptus "brought forth," hence "visible, apparent, evident," past participle of promere "to take or bring out or forth" (see prompt (v.)).Related: Promptly; promptitude.