- hay "grass mown," O.E. heg (Anglian), hieg, hig (W.Saxon) "grass cut or mown for fodder," from P.Gmc. *haujam (Cf. O.N. hey, O.Fris. ha, M.Du. hoy, Ger. Heu, Goth. hawi "hay"), lit. "that which is cut," or "that which can be mowed," from PIE *kau- "to hew, strike" (Cf. O.E. heawan "to cut;" see HEW (Cf. hew)). Slang phrase hit the hay (pre-1880) was originally "to sleep in a barn;" hay in the general figurative sense of "bedding" (e.g. roll in the hay) is from 1903.
Etymology dictionary. 2014.