To come back, and to give back. Both senses close a circuit that was opened.
Old French retorner (to turn back), from re- (again) + torner (to turn), from Latin tornare (to turn on a lathe); a circuit that closes, whether of a body coming back or a thing given back.
Both a coming back and a giving back — the closing of a circuit that was deliberately left open, whichever direction the closing runs.