late 14c., "interpretation of signs and omens," from Old French conjecture "surmise, guess," or directly from Latin coniectura "conclusion, interpretation, guess, inference," literally "a casting together (of facts, etc.)," from coniectus, past participle of conicere "to throw together," from com- "together" (see com-) + iacere "to throw" (see jet (v.)). Sense of "forming of opinion without proof" is 1530s.
early 15c., from conjecture (n.). In Middle English also with a parallel conjecte (n.), conjecten (v.). Related: Conjectured; conjecturing.