A close reference to "Waterhead" or "Smoothbrain". A "Marble Brain" is typically a just a stupid person with cognitive skills of elementary school kid. Every thought or response is simple & incoherent. Furthermore, it is believed a "Marble Brain" loses a single marble trying to repsond until all their marbles are gone. This is followed by rambling & annoying trailing sentences.
by Greendukey June 18, 2021