c. 1200, engoinen, "to prescribe, impose" (penance, etc.), from stem of Old French enjoindre (12c.) "impose (on), inflict; subject to; assign (to)," from Latin iniungere "to join, fasten, attach;" figuratively "to inflict, to attack, impose," from in- "on" (from PIE root *en "in") + iungere "to join together" (from nasalized form of PIE root *yeug- "to join"). Related: Enjoined; enjoining.

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."

in law, "a writ commanding something to be done or requiring a reason for its non-performance," c. 1500 (in Magna Carta in Anglo-Latin), from Latin praecipe, imperative of praecipere "to admonish, enjoin, take or seize beforehand," from the opening words of such a writ, praecipe quod reddat "enjoin (him) that he render." From prae (adv.) "before (see prae-) + -cipere, combining form of capere "to take," from PIE root *kap- "to grasp." 

"commanding officer," especially of a fortified town or garrison, 1680s, from French commandant "the one commanding" originally "commanding," present participle of commander (Old French comander) "to order, enjoin;" see command (v.). Similar formation in Spanish and Italian comandante.

"bordering on every side," late 15c., from Latin circumiacens, present participle of circumiacere "to border upon, to lie round about, enjoin," from circum "around, round about" (see circum-) + iacere "to throw, cast, hurl" (from PIE root *ye- "to throw, impel"). Related: Circumjacence; circumjacency.

"one who has the authority or power to command or order," early 14c., comandur, from Old French comandeor "commander, commandant," from comander "to order, enjoin" (see command (v.)). Commander in chief "commander of all the armies of a state" is attested from 1650s. In the U.S., by the Constitution, it is the president; George Washington was so called by 1778.

c. 1300, "order or direct with authority" (transitive), from Old French comander "to order, enjoin, entrust" (12c., Modern French commander), from Vulgar Latin *commandare, from Latin commendare "to recommend, entrust to" (see commend); altered by influence of Latin mandare "to commit, entrust" (see mandate (n.)). In this sense Old English had bebeodan.

Intransitive sense "act as or have authority of a commander, have or exercise supreme power" is from late 14c. Also from late 14c. as "have within the range of one's influence" (of resources, etc.), hence, via a military sense, "have a view of, overlook" in reference to elevated places (1690s). Related: Commanded; commanding.

Command-post "headquarters of a military unit" is from 1918. A command performance (1863) is one given by royal command.

c. 1300, "to unite (things) into a whole, combine, put or bring together; juxtapose," also "unite, be joined" (intrans.), from joign-, stem of Old French joindre "join, connect, unite; have sexual intercourse with" (12c.), from Latin iungere "to join together, unite, yoke," from nasalized form of PIE root *yeug- "to join."

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.