I know of the 2 different homonyms behind 'rejoin'; I ask only about the one that means 'to retort'.
rejoin (v.2) [⟸] "to answer," mid-15c., legal term,
from Middle French rejoin-, stem of rejoindre "to answer to a legal charge,"
from Old French re- "back" (see re-) + joindre "to join" (see join).
General (non-legal) meaning first recorded 1630s.
rejoinder (n.) [⟸] mid-15c., from Middle French noun use of rejoindre (see rejoin (v.2)). Originally "defendant's answer to the replication" (the fourth stage in the pleadings in an action at common law). For noun use of infinitive in French law terms, see waiver.
Please help me dig deeper than the definition, which I already understand and so ask NOT about. Please expose and explain all hidden, missing semantic drifts and links. How should the etymology be interpreted, to understand how the semantic drifts abstracted and severed from the original literal meaning?
1. How did re- "back" + joindre "to join" combine to mean the above?
What or who is joining back, to what, where, or whom?
2. The Modern French verb rejoindre doesn't possess the definition above. Does anyone know why?