Green’s Dictionary of Slang

(US) the Mayor, i.e. His Honor.

1931		P.J. Wolfson Bodies are Dust (2019) [ebook] ‘“Hizzoner” will be here. [...] All dressed up like a house on fire’.
1942		W. Winchell On Broadway 3 Jan. [synd. col.] Hizzoner admitted: ‘When I make a mistake it’s a beaut’.
1951		C. Stoker Thicker ’n Thieves 235: If Hizzoner was fighting crime, where was he fighting it?
2017		D. Winslow The Force [ebook] Hizzoner would fire the commish, except it would look like what it is.