Green’s Dictionary of Slang

1. a brash or boastful person.

1945		A.J. Liebling ‘Quest for Mollie’ in Just Enough Liebling (2004) 165: Mollie was the biggest popoff and the biggest screwball and the biggest foul-up I ever saw.
1953		L. Uris Battle Cry (1964) 292: I ain’t no pop-off.
1973		J. Wambaugh Onion Field 230: The beat cop had no reputation as a rollcall popoff. On the contrary, he was a quiet man.
1994		M. Ribowsky Don’t Look Back 90: Such priggishness—and not just by Stephens, who was known as a chronic pop-off—was an inverted form of Tomming.

2. an informer.

1935

A.J. Pollock Und. Speaks n.p.: Pop off, a stool pidgeon.

3. any form of comment.

1938

W. Winchell On Broadway 19 Dec. [synd. col.] Judge’s new reviewing policy. Hereafter, the mag will collect lobby pop-offs on the drama, to give it the audience angle.

4. attrib. use of sense 1.

1957

(con. 1944) A. Myrer Big War 243: Let’s not go making a liar and a pop-off character out of me now. Okay?