late 14c., "interpretation of signs, dreams, and omens," also "a supposing, a surmising," 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 assimilated form of com "together" (see con-) + iacere "to throw" (from PIE root *ye- "to throw, impel").
Sense of "an unverified supposition" is from 1520s; that of "act of forming of opinion without proof" is from 1530s.
early 15c., "infer, predict, form (an opinion or notion) upon probabilities or slight evidence," from conjecture (n.) or from verbs in Medieval Latin and Old French. Middle English also had parallel forms conjecte (n.), conjecten (v.). Related: Conjectured; conjecturing.