Green’s Dictionary of Slang

1. concern, affair; usu. in negative phr. none of one’s butt-in.

1902		C.L. Cullen More Ex-Tank Tales 47: I didn’t figure that it was my butt-in.
1964		J. Thompson Pop. 1280 (1990) 10: But that wasn’t none of my butt-in.

2. a meddler, one who interferes.

1905		H. Green Actors’ Boarding House (1906) 97: Me cookee. No want flesh butt-ins ’round!
1919		Sun. Times (Perth) 14 Sept. 25/4: Gaseous butt-ins have no right to argue as to how the cash should be cast around.
1936		Record (Hackensack, NJ) 30 Mar. 26/2: ‘You’re just a butt-in,’ said the Chief Executioner.
1975		(con. 1900s) G. Swarthout Shootist 75: It was some nobody, some butt-in with a secret compulsion to use a gun.

3. a gatecrasher.

1905		F. Hutchison Philosophy of Johnny the Gent 31: [He] breaks himself buyin’ Christmas presents fer [...] a few aunts an’ cousins an’ some more butt-ins that a feller forgets is on earth.
1911		S. Ford Torchy 92: There was only a few lady butt-ins that had strayed over from the shoppin’ district and smelled out a free show.