FuthaMukka is the correct spelling for the toned down version of MotherFucker. As a spoonerism FuthaMukka can be used with out offence in polite company or to avoid censorship.
It is rarely used as an insult as FuthaMukka is considered a complement when prefixed with words like bad, sexy, dirty etc.
Bad FuthaMukka, Sexy FuthaMukka, Dirty FuthaMukka
Play on words of the phrase m*ther f*cker
Also a range of clothing available to buy online.