Literal translation
he leads a life that only the good Lord knows
Meaning
he leads a life only God could account for
Explanation
The expression, seemingly discreet, hints through understatement that a person leads a disorderly, dissolute, or secretive life, without specifying its exact nature, as though only a higher power, in this case God himself, could know the actual details of such a life. This refusal to name the facts outright, while suggesting them insistently, belongs to a typical modesty of popular speech, which prefers a veiled hint to an open description of conduct deemed reprehensible or shameful. The verb saup (he knows), a form of saupre (to know), places God in the position of the only true witness to a reality that ordinary people can only imagine or suspect. The expression closely matches its French equivalent, both drawing on the same rhetorical device of religious understatement to evoke without stating. The register is familiar, tinged with quiet disapproval or a touch of humor depending on the tone used, employed to comment, with some restraint, on someone's questionable habits without going into detail.