Yours, Etc.: Origins and Uses of 8 Sign-Offs
1520s, originally a second crop of grass grown after the first had been harvested, from after + -math, a dialectal word, from Old English mæð "a mowing, cutting of grass" (see math (n.2)). Figurative sense by 1650s. Cf. French regain "aftermath," from re- + Old French gain, gaain "grass which grows in meadows that have been mown," from a Germanic source, cf. Old High German weida "grass, pasture"