Green’s Dictionary of Slang

pop off (at the mouth) v.

also pop off at
[SE pop, to explode sharply]

1. [19C+] to make a fuss about.

2. [1920s] to confess, to tell the truth.

3. [1930s–50s] to joke.

4. [1910s+] to talk in an enthusiastic or aggressive, threatening manner.

5. [1960s+] to brag, to boast.

6. [1980s] to criticize.