Yours, Etc.: Origins and Uses of 8 Sign-Offs
early 15c., from Latin dimissus, past participle of dimittere "send away, send different ways; break up, discharge; renounce, abandon," from dis- "apart, away" (see dis-) + mittere "send, let go" (see mission). Prefix altered by analogy with many dis- verbs. Dismit, in the same sense, is attested from late 14c. Related: Dismissed; dismissing.