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!
as defined by the reality TVshow Forensic Mysteries on HLN, this a synonym phrase for "not the brightest person around"
boy 1) yo come to my backyard, i want to start a party by going out to weed
boy 2) are you an uptard, your dad, or mom will notice.
boy 1) nope my mom is not even home from work and my dad is not the brightest lightbulb in the chandelier, he will think we are trying to burn Cheetos on the barbecue or something.