How did 'folding back' semantically shift to mean 'reply'?
To wit, how does the notion of "folding back" semantically appertain to "respond"?
Etymologically, reply means ‘fold back’. It comes ultimately from Latin replicāre ‘fold back, unfold’, a compound verb formed from the prefix re- ‘back’ and plicāre ‘fold’ (source of English ply and related to English fold). This came to be used metaphorically for ‘go over again, repeat’ (whence English replicate ), and also as a legal term for ‘respond’. In this latter sense it passed into English via Old French replier.
Word Origins (2005 2e) by John Ayto, p 421 Right column.
p 1785 on Oxford Latin Dictionary (2012 2 ed) doesn't expound the semantic shift. I marked the entry with a red line.