noun.

Person who exhibits the characteristics of being a plum (someone who's useless) and that of being a muppet (someone who's a bit stupid).

Due to the cute sounding nature of the word, it's usually reserved for someone you're friends with; calling a random nutter in the pub a pluppet will be greeted with him and his mates laughing at you, then pummelling you with a broken bottle of Becks.