Shit for brains actually come from the custom of Latvians who, at one time, used fecal matter to stuff a dead bodies crainial orifices before burial. This was an inexpensive method of getting rid of a lot of shit and saving a few bucks.
However, during WW2 GI Joes serving in Europe misunderstood the references and took it to mean an insult.
They began using the saying at home in the USA
Hey Gustov, you got shitska for brainskis? Please to be passing some over, I need for Uncle Batinov's funeral.
SHITSKA FOR BRAINSKIS translated: shit-for-brains.