Green’s Dictionary of Slang

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

1868		L.M. Alcott Little Women II 17: ‘I never cry unless from some great affliction.’ ‘Such as old fellows going to college, hey?’.
1887		‘’Arry on Ochre’ in Punch 15 Oct. 169/2: Save, hay, — out of two quid a week!
1979		E. Bond Worlds Part I iv: Hey up! old age P.
1998		(con. 1986) G. Pelecanos Sweet Forever 36: I’m older than you but, hey, I’m one of you.
1999		Guardian Guide 29 May–4 June 6: But, hey, I loved Men in Black.