c. 1300, "a communication transmitted via a messenger, a notice sent through some agency," from Old French message "message, news, tidings, embassy" (11c.), from Medieval Latin missaticum, from Latin missus "a sending away, sending, dispatching; a throwing, hurling," noun use of past participle of mittere "to release, let go; send, throw" (see mission).
The Latin word is glossed in Old English by ærende. Specific religious sense of "divinely inspired communication via a prophet" (1540s) led to transferred sense of "the broad meaning (of something)," which is attested by 1828. To get the message "understand" is by 1960.