Con*jec"ture (; 135?), n. [L. conjectura, fr. conjicere, conjectum, to throw together, infer, conjecture; con- + jacere to throw: cf. F. conjecturer. See Jet a shooting forth.] An opinion, or judgment, formed on defective or presumptive evidence; probable inference; surmise; guess; suspicion.

He [Herodotus] would thus have corrected his first loose conjecture by a real study of nature.
Whewell.

Conjectures, fancies, built on nothing firm.
Milton.

Con*jec"ture, v. t. [imp. & p. p. Conjectured (?); p. pr. & vb. n. Conjecturing.] [Cf. F. conjecturer. Cf. Conject.] To arrive at by conjecture; to infer on slight evidence; to surmise; to guess; to form, at random, opinions concerning.

Human reason can then, at the best, but conjecture what will be.
South.

Con*jec"ture, v. i. To make conjectures; to surmise; to guess; to infer; to form an opinion; to imagine.

• (countable or uncountable) A statement or idea which has not been proven, but is thought to be true; a guess
       I explained it, but it is pure conjecture whether he understood, or not.
  
• (intransitive) To guess; to venture an unproven idea.
        I don't know if it is true, I'm just conjecturing here.