punch her in the face
extremely inappropriate, yet i can't stop laughing :monsters-I was gonna suggest the back of the head.
Then you can also say that something hit her by accident. You won't fuck up her face, and she'll also have no reason not to cook you dinner when she regains consciousness.
The better question is, why are you even fucking your wife? Do you know how many easily-manipulated 9-year-olds are on the market right now? Christ.