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.