"My God, you know perfectly well"
It's an expression of frustration or surprise, often used when someone is aware of something that another person is trying to deny or avoid. It implies that the person knows the truth and is exasperated by their attempts to hide it.
Here's an example of how it might be used:
* Person A: "I didn't do it!"
* Person B: "Mon Dieu, tu sais parfait que c'est toi qui a cassé le vase!" (My God, you know perfectly well that you broke the vase!)