Green’s Dictionary of Slang

(US) a masturbator, thus an idiot, dolt.

1938		‘Justinian’ Americana Sexualis 26: Jagoff (Jackoff) A man who is presumed to masturbate excessively. Any male disliked because of his idiosyncrasies or unconventional social habits.
c.1948		‘Joe Stalin in The Great Leader’ [comic strip] in B. Adelman Tijuana Bibles (1997) 126: Stalin wants to fuck all Americans, and so do his men, the jag-offs.
1953		S. Bellow Augie March (1996) 101: Beat it, you little jag-offs [...] Damn snots, where’s your upbringing!
1955		E. Abbey Brave Cowboy (1958) 87: You swipes, you crumbs, you bloody jag-offs.
1997		Mad mag. Feb. 6: If even one of those jagoffs had even bothered to read it, they would have seen that this script is rancid.
1999		L. Gould Shagadelically Speaking 67: jagoff, Possible corruption of ‘jack off.’ An insult Scott Evil hurls at Klansman during a taping of The Gerry Springer Show.
1999		J. Ridley Love Is a Racket 158: Three fat, drunk jagoffs.
2001		J. Stahl Plainclothes Naked (2002) 179: Some fucked-up, insensitive jag-off was the one who knocked on your front door.
2001		OnLine Dict. of Playground Sl. 🌐 jagoff, jerkoff, jackoff n. derog. Regional term used heavily in the Chicago area. Refers to one who is useless, clueless, or generally an idiot. e.g. ‘The guy in the ’89 Mustang with the mullet is such a jagoff’.
2004		Mad mag. May 8: No jag off has cut in front of me in a bank line.
2011		J.J. DeCeglie Drawing Dead [ebook] I’d told her to go and blow the jag-off behind the desk a big soul kiss.
2019		S.M. Jones Lives Laid Away [ebook] ‘I was at the top of the stairs polishing some jagoff’s knob from the State’s Attorney General’s office’.