A Freudian camisole is a variation on the common phrase "Freudian slip." A Freudian slip is when you accidentally say something that reveals something about your subconscious mind. In contrast, a Freudian camisole is when you accidentally hear something wrong. This may or may not reveal something about your subconscious.
(A camisole is a type of undergarment, like a slip. This is a pun.)
Girl: And that's where the road intersects-
Boy: *not paying attention* Wait did you say sex?
Girl: What? No. You heard me wrong. Must have been a Freudian camisole...