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
b) i.e. "Silly billy", "You big dafty"
c) "That numpty's driving with no lights on!"