A dumb fuck is someone that is undeniably stupid, they can't get their life together because of their epic level of stupidity, so they just continue to do dumb fucking shit,
like dumping their millionaire
girlfriend for an old Filipino prostitute that's married and has a
kid.
(A) Have you ever met
that guy Toby?
(B) No, I don't think I have...wait! Is he a dumb
fuck?
(A) Yes! He's the guy I was telling you about, the one who got engaged to a married Filipino prostitute.
(B) Woah, he really is a dumb
fuck.