"to answer," 1447, legal term, from M.Fr.
rejoin-, stem of
rejoindre "to answer to a legal charge," from O.Fr.
re- "back" +
joindre "to join" (see
join). General (non-legal) meaning first recorded 1637.
Rejoinder is 1450, from M.Fr.
rejoindre; originally "defendant's answer to the replication."