Green’s Dictionary of Slang

1. to make a fuss about.

1909		Northern Automotive Jrnl 18-19 47: To get swelled up with anger and pop off at the mouth is like steam blowing off at the safety valve of the boiler. It is wasted energ.
1935		E. Anderson Hungry Men 111: ‘You are getting smart now.’ ‘Forget it,’ Acel said. ‘I’m just poppin’ off. It’s none of my business.’.
1953		W.P. McGivern Big Heat 76: Keep those people in mind [...] when you’re popping off about your own serene little corner of Heaven.
1967		(con. 1920s) J. Thompson South of Heaven (1994) 157: Have you been popping off around camp about this?

2. to confess, to tell the truth.

1929

D. Hammett ‘Fly Paper’ Story Omnibus (1966) 37: Pop off, you lightweight, or I will.

3. to joke.

1938		W. Winchell On Broadway 28 Nov. [synd. col.] William Lyon Phelps pops off: ‘For a sense of victory instead of failure and defeat, readers might turn to the sports pages.’.
1952		J. Jones From Here to Eternity (1998) 580: As a joke, it did not come off very well. [...] ‘Shut up, Mack,’ the Sgt said to Prew. ‘The more you pop off, the harder you make it for on yourself.’.
1960		J. Brosnan Long Season 44: ‘Maybe you ought to try learning something instead of poppin’ off, young man’.

4. to talk in an enthusiastic or aggressive, threatening manner.

1914		Spanish Amer. (Roy, NM) 24 Jan. 12/1: Certain events [...] have grated our nerves [...] it would be very easy for us to ‘pop off’ and say things which — though true — might get us in bad.
1923		Honolulu Adveriser (HI) 6 July 11/6: Even the coach of the defeated Hilo crews pops off loudly about Nicholl’s ability.
1931		E.M. Berry ‘Sawmill Divertissements’ in Botkin Folk-Say 202: A man that has to git up at five-thirty hasn’t got any business listenin’ to me pop off.
1943		M. Curtiss Letters Home (1944) 8 Sept. 247: I’ve seen many of these tough babies shaking like leaves because of things that don’t phase us quiet fellows who aren’t always popping off about how tough we are or what we’ll do to the enemy.
1943		W. Guthrie Bound for Glory (1969) 100: Quit popping off at your mouth!
1953		J. Thompson Criminal (1993) 73: I was just popping off.
1961		T.I. Rubin In the Life 93: You know me and my mouth—it forgets I’m there and starts popping itself off. [...] my yapper keeps doing a big sales pitch.
1971		T. Thackrey Thief 415: I got sort of rowdy [...] I guess I popped off quite a bit.
1998		D. Remnick King of the World 157: Now here comes Cassius Clay popping off and abusive and loud.
2000		Eminem ‘Kill You’ 🎵 You better kill me! I’ma be another rapper dead / for poppin off at the mouth with shit I shouldn’ta said.

5. to speak critically.

1939		Goodman & Kolodin Kingdom of Swing 114: As I look back on it now, I must have been a fresh kid, popping off .
1989		P. Munro Sl. U. 150: I don’t know why my dad is always popping off about my clothes.

6. to brag, to boast.

1962		P. Highsmith Cry of the Owl (1968) 256: What do you mean by popping off to every dope who looks at you that I kept you in New York?
1964		J. Thompson Pop. 1280 in Four Novels (1983) 452: It sure ain’t my fault if Ken [...] pops off all over town about what a tough fella he is.
1989		P. Munro Sl. U.