also re-join, 1540s, of things, intransitive, "unite again, unite after separation," from re- "again" + join (v.). Transitive sense of "join again, reunite (one thing or person to or with another)" is from 1560s. Meaning "join the company of again" is from 1610s. Related: Rejoined; rejoining.
"to say in answer to a reply or later remark," mid-15c., a legal term, "answer a reply, reply a second time" (to a charge or complaint), from Old French rejoin-, stem of rejoindre "to answer to a legal charge," from Old French re- "back" (see re-) + joindre "to join, connect, unite," from Latin iungere "to join together, unite, yoke" (from nasalized form of PIE root *yeug- "to join").