Green’s Dictionary of Slang

dirtbag n.

[SE dirt + -bag sfx]

1. (orig. US, also dirthead) a general term of abuse, irrespective of sex.

[[US]P. Kendall Army and Navy Sl. 4: Dirt bag [...] a soldier on the garbage detail].
[US]Baker et al. CUSS 106: Dirt bag An obnoxious person.
[UK]D. Gram Foxes (1980) 82: ‘You guys all think you’re so hip, slick and cool,’ Scott said with a sneer. ‘Well, you’re all dirt heads, you know that?’.
[US]C. Hiaasen Skin Tight 24: Guy sounds like a dirtbag, anyway.
[US]R. Marcinko Rogue Warrior (1993) 252: What sort of people can slide in the back door most easily? Dirtbags.
[US]N. Green Shooting Dr. Jack (2002) 274: That guy’s a dirtbag.
[Aus]R.G. Barrett Mystery Bay Blues 18: If ever he came across an old lady being mugged and caught the dirtbag doing it [etc.].
posting at Channel-4 TV News 6 May [Internet] Charles is a two-timing filandering dirtbag.
[US]C. Stella Rough Riders 36: The world gets to deal with X amount of dirt bags to balance out the good people.
[US]D. Winslow The Force [ebook] Malone sitting on a steel bench in a holding cell like the dirtbags he usually puts there.

2. a promiscuous woman.

[Aus]B. Moore Lex. of Cadet Lang. 117: usage: What — you fucked that dirtbag? Shit! — you must have been desperate.