Green’s Dictionary of Slang

hey! excl.

also hay!
[SE hey!, a greeting]

a piece of verbal punctuation, used as a form of intimacy, suggesting a complicity between addressee and addressed.

[US]L.M. Alcott Little Women II 17: ‘I never cry unless from some great affliction.’ ‘Such as old fellows going to college, hey?’.
[UK] ‘’Arry on Ochre’ in Punch 15 Oct. 169/2: Save, hay, — out of two quid a week!
[UK]E. Bond Worlds Part I iv: Hey up! old age P.
[US](con. 1986) G. Pelecanos Sweet Forever 36: I’m older than you but, hey, I’m one of you.
[UK]Guardian Guide 29 May–4 June 6: But, hey, I loved Men in Black.

In phrases

make hey-hey (v.) (also make hay-hay)

(US) to act in a boisterous, celebratory manner.

[US]Blanche Calloway ‘Catch On’ 🎵 When summer is here, / the farmers get gay, / They all make whoopie and hay-hay!
[UK]P. Cheyney Dames Don’t Care (1960) 67: There are [...] some city guys from Los Angeles makin’ hey-hey.