An expression to someone that you think they are utterly stupid & probably wouldn't compare to a retarded lizard.
Brian you are such a dunce, GO SIT IN THE CORNER!
One who is very stupid - a fuckwit.
Dunce originates from when children who refused to learn or just bloody stupid and sent to the corner and wore a giant cone with a red D for dunce on it.
Timmy you fucking dunce to the corner!