late 14c., "a joining or meeting of individuals or distinct things," originally of planets or stars "meeting" in the same part of the sky, from Old French conjonction "union, joining, sexual intercourse" (12c.), from Latin coniunctionem (nominative coniunctio), noun of action from past-participle stem of coniugare "to join together," from assimilated form of com "with, together" (see con-) + iugare "to join," from iugum "yoke" (from PIE root *yeug- "to join").
Compare Italian congiunzione, Spanish conjunción, from the same Latin noun. The English word also had the meaning "sexual union" 17c.-18c. Old English used geðeodnys as a loan-translation of Latin coniunctio.
Grammatical sense of "connective particle serving to unite clauses of a sentence or coordinate words in a clause or sentence" (late 14c.) was in Latin, a loan-translation of Greek syndesmos.