c. 1300, "unite (something to something else), ally" (a sense now obsolete); late 14c. as "be contiguous with, be adjacent to," from Old French ajoin- stem of ajoindre "join together, unite," from Latin adiungere "fasten on, harness, join to," from ad "to" (see ad-) + iungere "to bind together," from a nasalized form of PIE root *yeug- "to join." Meaning "be contiguous with, be in contact with" is from late 14c. Thje French word was Latinized 16c. to Modern French adjoindre. Related: Adjoined; adjoining.