"grass mown," Old English heg (Anglian), hieg, hig (West Saxon) "grass cut or mown for fodder," from Proto-Germanic *haujam (source also of 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;" source also of 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 1941.
also hay-fever, 1825, from hay + fever. Also called summer catarrh (1828); not much noted before the 1820s, when it was sometimes derided as a "fashion" in disease.
People are apt to sneeze, in hot weather for example; and people do not die of sneezing now-a-days, as they did in days that no one knows any thing about. We cannot give six draughts a-day, at one and nine pence each, for sneezing: call it the hay-fever. What a wonderful man! what a clever man! he understands the hay-fever: call him in! Thus is the hay-fever among the last in the list of fashionables. ["On Fashions in Physic," London Magazine, October 1825]
also hay-seed, 1570s, "grass seed shaken out of hay," from hay + seed (n.). In U.S. slang sense of "comical rustic" it dates from 1875. To have hayseed in (one's) hair was a common mid-19c. way in U.S. to indicate a country person.
The opinion of the court was delivered by Justice Hunt; the chief justice, in whose hair the Ohio hayseed still lingers, delivering a dissenting opinion (etc.) [The Chronicle, New York, Nov. 12, 1874]
"stack of hay," Old English muga, muwa "a heap (of grain, pease, etc.), swath of corn; crowd of people," earlier muha, from Proto-Germanic *mugon (source also of Old Norse mugr "a heap," mostr "crowd"), of uncertain origin. Meaning "place in a barn where hay or sheaves of grain are stored" is by 1755.