early 15c., legal, "a charge, a formal accusation," from Old French surmise "accusation," noun use of past participle of surmettre (see surmise (v.)). Meaning "inference, guess" is first found in English 1580s.
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez, when with eagle eyes
He stared at the Pacific—and all his men
Look'd at each other with a wild surmise—
Silent, upon a peak in Darien.