I dont remember anything about the plot except that the assassin is a obese middle aged lady in a laura ashley dress and a shopping bag. she even pretends to be her own go-in-between , gets a young guy to screw her and throws her dryer in his shower.

I was watching a movie the other day which had an unlikely assassin who nobody suspects which reminded me of this book. My head is about to explode! plz help!

I believe you are talking about Windmills Of the Gods by Sidney Sheldon.