Literal translation
he knows where Jeanne sleeps
Meaning
he knows every nook and every secret
Explanation
This expression means that someone knows every nook and every secret of a place or a matter, especially the location of a hiding place where, figuratively, a hoard or a sum of money lies asleep. It is made up of well attested words: to know, where, to sleep, and the first name Jeanne, used here as a conventional figure, perhaps the mistress of the house or a female figure traditionally linked to guarding the family's wealth. The image plays on the idea that money, like a sleeping person, rests hidden somewhere, and that only the initiated know the exact spot of that rest. It is used to speak of someone with privileged knowledge of a household's savings, or more broadly of someone who knows the ins and outs of a situation perfectly, its hidden resources as well as its weaknesses. The register is casual, tinged with a touch of mischief, fit for remarks between close acquaintances about someone else's money matters.