A stupid person. Because the word isn't in great usage right now, referring to a person as a "nitwit" can be considered a more intense way of insulting them.
Tommy: "Cole, you stupid."
Cole: "Eh, whatever, man."
*application of the term*
Tommy: "Cole, you're a complete nitwit."
Cole: "Who do you think you're talking to?"
The Urban Dictionary Mug
One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.
Buy the mug