Both my children use the word 'numpty' a lot to describe a silly person. The OED says it is British, originally Scottish, slang. It's quite a recent word, the first recorded instances being in the 1980s. The origin is uncertain but it might have come from the much older (16th century) numps, which meant the same thing and may originally have been a pet name for Humphrey. Numpty may also have developed from the word numbskull, which was once spelled numpskull and numpscull. There is also the Wiltshire dialect word naumpey, which means 'weak and foolish-minded person'.