Numpty:
1)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.
b) A good humoured admonition, a term of endearment
c) A reckless, absent minded or unwise person a) "No. That wisnae wit she meant, ya big numpty!"
b) i.e. "Silly billy", "You big dafty"
c) "That numpty's driving with no lights on!"
2)Dialect, chiefly scots. A bumbling fool or one who is intellectually challanged.
(plural : numpties) "they numpties couldnae organise a pissup in a brewery."
Selected bits from the Urban Dictionary. Other humorous, but questionable entries were edited out.