Green’s Dictionary of Slang

(US) stupid.

1986		L. Heinemann Paco’s Story (1987) 153: Back when those jag-off housecats from CBS and NBC [...] didn’t get much past the far end of the lobby bar at the Honolulu Hilton.
2011		J.J. DeCeglie Drawing Dead [ebook] I blew a hundred [...] betting random horses on a jag-off run of epic fuckhead proportions.
2017		T. Pluck Bad Boy Boogie [ebook] ‘That was a real jag-off move’.