Re: The ultimate MitM-quotes Quiz
I think it's from Houseboat (3x01), when Malcolm came back from fishing with Hal. Reese and Stevie spent their day with girls, and they kept a bra.
During dinner, they showed it to Malcolm, who answered this.
(then Reese smelt it to prove it didn't belong to their mother)
I've only seen this episode with french dubbing, but I think it comes from it.