I saw this word in a comment on a blog and it made me laugh so much. It's one of those words. You hear it, and you don't need an explanation. You know what it means. In fact, you're probably thinking of someone who is a twazmuppet right now.
I've been experiencing some schadenfreude of late. I know I'm a bad person, but I love to say 'I told you so'. There's a little part of me that likes to be proved right by subsequent events. Especially when people were behaving like twazmuppets and now their behaviour is catching up with them.
I think another reason I like this word is because it catches that moment when you're about to call someone a 'twat', then catch yourself because that's not a very Christian word to use (apparently), and you have to supplant the final T with another word entirely.
Like this: "Oh, he's such a twaaa-zzmuppet."