Mugwump has its own Wikipedia entry, daisy. Check it out! It's not a word I use, but I have come across it before. I think the British equivalent might be "numpty", which I hear used a lot; but I don't use that either.
A word that makes my wife and me smile is "supposably", and we often use it between ourselves. We came across it in an episode of "Friends", when Chandler told Joey he would never go out with a girl who said "supposably". It was Joey's delayed reaction to the word that tickled our fancy, and still does. (I've just found the one-minute clip on YouTube, if anybody's interested. Just search for "Joey Friends supposably".)
I love the word "penguin". As a writer, I am waiting for the day I can write a story with these lines: It was early evening when Gwen and I arrived at the library. I could hardly wait to sign my name to the list and win the million dollars. I raced to the counter, reached into my pocket only to stumble away in horror. "I can't find my lucky pen, Gwen," I whispered.