Green’s Dictionary of Slang

repellent, disgusting, despicable.

1986		C. Hiaasen Tourist Season (1987) 20: ‘Tell me about Ernesto Cabal.’ ‘Dirtbag burglar.’.
1990		J. Wambaugh Golden Orange (1991) 50: Uses meth and other dirtbag drugs.
2002		J. Lerner You Got Nothing Coming 118: Dick-licking dirtbag Dirtboys!
2016		Guardian CiF 10 Dec. 🌐 It’s the dirtbag left, who have had enough of privately-educated nonsense merchants wibbling on about progress.
2017		Tablet 9 Jan. 🌐 It also doesn’t mean aping Trump’s demagoguery with a left-wing sheen, as dirtbag leftists would have it.