Surmise, n.
Etymology: [OF. surmise accusation, fr. surmettre, p.p. surmis, to impose, accuse; sur (see Sur-) + mettre to put, set, L. mittere to send. See Mission.]
1. A thought, imagination, or conjecture, which is based upon feeble or scanty evidence; suspicion; guess; as, the surmisses of jealousy or of envy. [We] double honor gain From his surmise proved false. Milton. No man ought to be charged with principles he actually disowns, unless his practicies contradict his profession; not upon small surmises. Swift.
2. Reflection; thought. [Obs.] Shak.
Syn. -- Conjecture; supposition; suspicion; doubt.
1 other Webster entry under this spelling
Surmise, v. t. [imp. & p. p. Surmised; p. pr. & vb. n. Surmising.]
To imagine without certain knowledge; to infer on slight grounds; to suppose, conjecture, or suspect; to guess. It wafted nearer yet, and then she knew That what before she but surmised, was true. Dryden. This change was not wrought by altering the form or position of the earth, as was surmised by a very learned man, but by dissolving it. Woodward.
Shades of meaning
Fernald has no entry of its own for surmise; he lists it under doubt, hypothesis, suppose.
Where he writes about it:
A conjecture is more methodical than a guess, while a supposition is still slower and more settled; a conjecture, like a guess, is preliminary and tentative; a supposition is more nearly final; a surmise is more floating and visionary, and often sinister; as, a surmise that a stranger may be a pickpocket. — from hypothesis