When a man dresses up as a woman, but is not really a transvestite. The reference is from the movie "Mrs. Doubtfire" in which Robin Wiliams dressed up as a woman to get a job working as a nanny for his own children.
That pervert Bobby has been doubtfiring his way into the women's locker room at the gym...
Al: Now that we've weeded out another traitor from the ranks, let's see if there are anymore Mrs. Doubtfires in here!
(pulls on the hair of a powerfully built woman and is surprised to discover it really is a woman)
Al: Hey! You're a woman!