"transmit further on or back again," 1868, from re- "back, again" + transmit (v.). Related: Retransmitted; retransmitting.

c. 1400, relaien, "to set a pack of (fresh) hounds after a quarry;" also "change horses, take a fresh horse," from Old French relaiier, from relai (see relay (n.)). The word seems to have faded out by 19c. but was re-formed in electromagnetics from the noun, in a transitive sense of "pass on or retransmit," originally of telephone signals (1878), later in a transferred sense of "pass on information" (by 1956). Related: Relayed; relaying.