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.
I found a girlfriend, but she's got a kid, but its ok because she sends him out to play when I come over - You motherfucker.
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