Er soll vor der eigenen Tür kehren
means 'He should mind his own business'. (Of course, I use 'he' here, but any other pronoun works just as well.) However, its literal meaning is 'He should sweep in front of his own door'
What is the origin of this idiom? How did the idea that one should sweep in front of one's own front door turn into an idiom about minding one's own business?