Meaning "unite, become associated, form an alliance" is from early 14c. Meaning "to unite (two persons) in marriage" is from mid-14c. Figuratively (of virtues, qualities, hearts, etc.) from late 14c. Of battles, "to begin," from late 14c. In Middle English join on (c. 1400) meant "to attack (someone), begin to fight with." Meaning "go to and accompany (someone)" is from 1713; that of "unite, form a junction with" is from 1702. Related: Joined; joining.
Join up "enlist in the army" is from 1916. Phrase if you can't beat them, join them is from 1953. To be joined at the hip figuratively ("always in close connection") is by 1986, from the literal sense in reference to "Siamese twins." In Middle English, join sometimes is short for enjoin.
Proto-Indo-European root meaning "to join."
It forms all or part of: adjoin; adjust; conjoin; conjugal; conjugate; conjugation; conjunct; disjointed; enjoin; injunction; jugular; jostle; joust; join; joinder; joint; jointure; junction; juncture; junta; juxtapose; juxtaposition; rejoin (v.2) "to answer;" rejoinder; subjoin; subjugate; subjugation; subjunctive; syzygy; yoga; yoke; zeugma; zygoma; zygomatic; zygote.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit yugam "yoke," yunjati "binds, harnesses," yogah "union;" Hittite yugan "yoke;" Greek zygon "yoke," zeugnyanai "to join, unite;" Latin iungere "to join," iugum "yoke;" Old Church Slavonic igo, Old Welsh iou "yoke;" Lithuanian jungas "yoke," jungti "to fasten to a yoke;" Old English geoc "yoke."
late 14c., "to join together, unite; form a union or league," from Old French conjoindre "meet, come together" (12c.), from Latin coniungere "to join together," from assimilated form of com "with, together" (see con-) + iungere "to join together" (from nasalized form of PIE root *yeug- "to join"). Related: Conjoined, conjoining. Conjoined in reference to "Siamese twins" is recorded from 1749.