From
urbandictionary.com : (link has an 18 certificate...
)
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!"
Regards,
Chris.