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!"
A person who is incapable of performing the simplest of task correctly. Usual symptoms include poor hand-eye coordination, zero common sense and the generalillusion that they are special (caused by parents referring to them as such and the numpty not fully grasping the meaning implied).
Common causes are being dropped as a baby or repeated heavy blows to head in later life.
person1: Jeff could you pass me the pen?
Jeff: ooops! (while droppingthe pen)
Person1: Jeff you're such a numpty!