Green’s Dictionary of Slang

(US) an avenue.

1885		‘High School Sl.’ in N.Y. Dispatch 31 May 7: I just met Mac Downley down on the av.
1919		O.O. McIntyre New York Day by Day 10 Jan. [synd. col.] A group of Park Row news hounds [...] galloped up to Mrs Vincent Astor’s home on Fifth av.