hay (n.) Look up hay at Dictionary.com
"grass mown," Old English heg (Anglian), hieg, hig (West Saxon) "grass cut or mown for fodder," from Proto-Germanic *haujam (cognates: 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" (cognates: 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" is from 1903; roll in the hay (n.) is from 1945.