Not the brightest bulb in the chandelier

Someone slow, dimwitted or stupid - another way of saying "Not the sharpest knife in the drawer". Also, someone who stands out in a crowd as being particularly unintelligent.

Marisa: Why does Eric even bother talking in class - he has no idea what he's talking about.

Michael: What do you expect? I mean he's not the brightest bulb in the chandelier!