What is the plural of muff?
The plural of muff is muffs, e.g. the girls had lovely muffs to keep their hands warm.
A muff is actually a fur tube that people used to wear on their hands to keep them warm -- they stuck one hand in each end of the muff. Ear muffs are furry circles that keep the ears warm. The slang meaning is of the "furry" part of a woman -- in other words, her genital area. "Muff diving" means oral sex on a woman.
Tom felt bad for Muff Potter because he had witnessed Muff being framed for murder by Injun Joe. Tom wanted to feel like he was giving back to Muff in order to cover up his guilt for keeping quiet about Injun Joe. That's all I'm going to say, since I'm assuming that you are asking a question directly from your reading homework. :)