The phrase is often thought to refer to a person who performs the universal taboo of fucking his own mother, meaning he is unsrcupulous.
With regards to a child's welfare, it can be seen as refering to one who has sex with a single woman who has a child. If one fucks the child's mother, one is also a motherfucker.
Black slang term derived from when the overseers and slave drivers were encouraged to breed with the field negroes in order to produce a naturally less smelly house servant.
Quimbo: That ned sho be a muddafucker (motherfucker) bad man he don it well...bend ova da table doody booty! Heeee heeeeee!