This one goes way back, probably to New England Yankees. It actually means someone with an inflated opinion of himself who lets the world know about it--a windbag. It also has an undertone of meaning someone who's basically an unworthy lazy slug. Not a pretty picture!
A term of derision that began to develop in the 1950s. A person who is held in contempt by others for making poor decisions or for being generally ignorant.
"You pus-brain! You're supposed to throw water on the fire to put it out, not gas!