"grass mown," Old English heg (Anglian), hieg, hig (West Saxon) "grass cut or mown for fodder," from Proto-Germanic *haujam (cf. Old Norse hey, Old Frisian ha, Middle Dutch hoy, German Heu, Gothic hawi "hay"), literally "that which is cut," or "that which can be mowed," from PIE *kau- "to hew, strike" (cf. Old English heawan "to cut;" see 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.
In bed, either sleeping or cavorting: Joe's in the hay, zonked out/ a toss in the hay with her boss
[1940s+; probably fr hit the hay, found by 1912]
properly so called, was not in use among the Hebrews; straw was used instead. They cut the grass green as it was needed. The word rendered "hay" in Prov. 27:25 means the first shoots of the grass. In Isa. 15:6 the Revised Version has correctly "grass," where the Authorized Version has "hay."