hey interj.

c. 1400 as part of the refrain of a song; by 1550s as an exclamation to express yawning, sighing, etc.; see hey.

also hey-day, late 16c. as an exclamation, an alteration of heyda (1520s), an exclamation of playfulness, cheerfulness, or surprise something like Modern English hurrah; apparently it is an extended form of the Middle English interjection hey or hei (see hey). Compare Dutch heidaar, German heida, Danish heida. Modern sense of "stage of greatest vigor" first recorded 1751 (perhaps from a notion that the word was high-day), and it altered the spelling.

exclamation of greeting, 1862, American English (first recorded reference is to speech of a Kansas Indian), originally to attract attention (15c.), probably a variant of Middle English hy, hey (late 15c.) which also was an exclamation to call attention. The only definition in the "Century Dictionary" [1902] is "An exclamation of surprise, admiration, etc.: often used ironically and in derision," suggesting the development as a greeting-word mostly took place early 20c.

Even more informal is the widely used 'Hi.' A friendly greeting for people who already know each other, it should never be said in answer to a formal introduction, but it is universally used, and accepted, by the young. ["The New Emily Post's Etiquette," 1922]

Extended form hiya attested from 1940.