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 term used to reference cannabis/marijuana/weed.

A person who is incapable of performing the simplest of task correctly. Usual symptoms include poor hand-eye coordination, zero common sense and the general illusion 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.

A person who never has or never will have a f*ing clue what he is doing

An idiot, a silly person

Dialect, chiefly scots. A bumbling fool or one who is intellectually challanged.
(plural : numpties)

a scottish term, short for numb-skull.
idiot
pratt
wally