Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1200, variously, in Middle English, hei, hai, ai, he, heh, expressing challenge, rebuttal, anger, derision, sorrow, or concern; also a shout of encouragement to hunting dogs. Possibly a natural expression (cf. Roman eho, Greek eia, German hei).
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.
An exclamation used to underscore mildly what is said: Pennzoil has been arguing that, hey, they are reasonable people/ I tried explaining that, hey, basically a goose is just a big duck/ Hey, I'm only human
[1980s+; the use is attenuated from the ancient call for attention found by 1225]