An Etymological Dictionary of Astronomy and Astrophysics

1a) An opinion that one reaches or to which one commits oneself on the basis of probability alone or in the absence of any evidence whatever.

1b) The act of forming such an opinion.

2a) To arrive at or commit oneself to an opinion about (something) without having sufficient evidence to support the opinion fully.

2b) To estimate or conjecture about correctly (Dictionary.com).

Etymology (EN): M.E. gessen, perhaps from Scandinavian; cf. Sw., Dan., Norw. gissa, M.L.G. gissen, M.Du. gessen, related to verb get.

Etymology (PE): Hads, from Ar. Hads “intuition, conjecture, surmise.”