1540s, "pertaining to marriage, nuptial," also "pertaining to the relationship of husband and wife," from Middle French conjugal (13c.), from Latin coniugalis "relating to marriage," from coniunx (genitive coniugis) "spouse," which is related to 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"). Related: Conjugacy; conjugality.