1350-1400;Middle Englishsurmisen < Anglo-Frenchsurmis(e), Middle French (past participle of surmettre to accuse < Latinsupermittere to throw upon), equivalent to sur-sur-1 + mis (masculine), mise (feminine) < Latinmissus, missa, equivalent to mit(tere) to send + -tus, -ta past participle suffix
c.1400, "to charge, allege," from Old French surmis, past participle of surmettre "to accuse," from sur- "upon" (see sur-) + mettre "put," from Latin mittere "to send" (see mission). Meaning "to infer conjecturally" is recorded from 1700. Related: Surmised; surmising.
early 15c., legal, "a charge, a formal accusation;" see surmise (v.). Meaning "inference, guess" is first found in English 1580s.