n. Brit. sl. - A stupid person, a fool. (16th c: orig. penis.) - The Oxford English Dictionary
'Pillock' has become something of an accepted word in today's modern British society, even used by politicians on television without fear of reprisal from angry parents. It is one of those vaguely humourous Englishisms like 'prat' or 'cobblers' that originates from the favourite source for all slang - genitalia - but which, also like the above, has lost its meaning over time and become part of ordinary conversation.
Pillock originates from the Scottish word 'pillicock', meaning 'penis' and dating from the 1600s. (It can be found in King Lear : Act 3, Scene 4, though whether this is the first recorded usage of the word I do not know.) It is more commonly used in northern parts of Britain than in the south, and as yet, does not seem to have invaded the consciousness and language of any other country.
'You've gone and dropped it, you daft pillock!'