c.1410, "hounds placed along a line of chase," from M.Fr. relai
"reserve pack of hounds or other animals" (13c.), from O.Fr. relaier
"to exchange tired animals for fresh," lit. "leave behind," from re-
"back" + laier
"to leave" (see delay
). The etymological sense is "to leave (dogs) behind (in order to take fresh ones)." Of horses, 1659. Electromagnetic sense first recorded 1860. As a type of foot-race, it is attested from 1898. The verb is first attested c.1410.