numpty: (nump-tì). Scottish usage.
a) Someone who (sometimes unwittingly) by speech or action demonstrates a lack of knowledge or misconception of a particular subject or situation to the amusement of others.
"No. That wisnae wit she meant, ya big numpty!"
b) A good humoured admonition, a term of endearment
i.e. "Silly billy, you big numpty"
c) A reckless, absent minded or unwise person
"That numpty's driving with no lights on!"
But my favourite definition is:
numpty: A person who never has or never will have a fucking clue what he is doing. — Bobby Gould
From the Urban Dictionary.