Hey
natural exclamation: first recorded in Middle English.
wiktionary
From Middle English hey, hei, also without h- in ey, from Old English *hē, ēa(interjection), attested as first element in hēlā, ēalā(“O!, alas!, oh!, lo!”). Cognate with Dutch hé, hei(“hi, hey”), German hei(“hey, wow”), Danish and Swedish hej(“hello, hey”), Faroese hey(“hey, hello”), Old Norse, Icelandic and Norwegian hei(“hey”), Romanian hei, Russian эй(ej, “hey”); see heigh. Probably a natural expression, as may be inferred from its presence with similar meaning in many other unrelated languages: for example, Burmese ဟေး(he:), Finnish hei, Unami hè, and Mandarin 哎(āi), and various sound-alikes as Ancient Greek εἶα(eîa) and Latin eia, eho, Sanskrit हे(he). See also hello.
From French haie(“hedge”), with reference to the weaving patterns used in hedgelaying.
See he.
etymonline
hey (interj.)
c. 1200 as a call implying challenge, rebuttal, anger, derision; variously spelled in Middle English hei, hai, ai, he, heh. Later in Middle English expressing sorrow, or concern; also a shout of encouragement to hunting dogs. Possibly a natural expression (compare Roman eho, Greek eia, German hei, Old French hay, French eh). In modern use often weakened, expressing pleasure, surprise.
Þa onswerede þe an swiðe prudeliche, `Hei! hwuch wis read of se icudd keiser!' ["St. Katherine of Alexandria," c. 1200]
In Latin, hei was a cry of grief or fear; but heia, eia was an interjection denoting joy.
