A vague insult hinting at the target's mental inability. Since nearly all insults are used interchangeably as expressions of overblown fury designed to humiliate or provoke retaliation, the exact meanings of any and all insults (as well as their applicability) are completely irrelevant, although they commonly make jabs at either the subject's difficulty managing cogent thought processes or his sexual performance/orientation. The common suffixes "fuck" or "shit" are often appended in the form of a surrogate adjective denoting the seriousness of the offense. A "dumbshit" person is therefore more dumb than a merely "dumb" person because of the presence of the "shit" modifier. As such, all you idiots inventing and quibbling over precise definitions are wasting your time and mine. Worse still are the dumbshits giving thumbs up and down.
Look at that dumbshit. Look at that dumbshit dance.