join (v.)

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.

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Definitions of join from WordNet
join (v.)
become part of; become a member of a group or organization;
He joined the Communist Party as a young man
Synonyms: fall in / get together
join (v.)
cause to become joined or linked;
join these two parts so that they fit together
join (v.)
come into the company of;
She joined him for a drink
join (v.)
make contact or come together;
The two roads join here
Synonyms: conjoin
join (v.)
be or become joined or united or linked;
Our paths joined
Synonyms: connect / link / link up / unite
join (n.)
the shape or manner in which things come together and a connection is made;
join (n.)
a set containing all and only the members of two or more given sets;
Synonyms: union / sum