mutha fwaking or mutha fwacker is a way to say mother fucking or mother fucker in a swear filter or around people you can't/won't/ shouldn't swear (cuss) around.
Me: This ****** ******* app is ridiculous
Somebody Replying: Dude, it filtered you!!!
Me: Fawk this app. It's mutha fwaking ridiculous......
Some douche bag: Bro you stupid jus get off the app
Me: Stfu ****** ******
Me: AGH, mutha fwaker!!!!!