So I was reading Freud's book A General Introduction to Psychoanalysis because I had a particularly vexing question about the practice of psychology and went straight to the best source of information. Just kidding! Freud ruined psychology. More like Sigmund Fraud! HAH!
But for serious, I rescued the book from my parents' garage sale, and began to flip through it. The first page I read was p. 169, where the following passage appears:
"There is a well-known joke in which an intelligent Jewish boy, when asked who was the mother of Moses, answers immediately: 'The Princess.' He is told: 'No, she only took him out of the water.' 'That's what she said,' he replies."
Now I admit, I don't really get it. Either the myth of Moses is something I don't know enough about for it to make sense, or "took him out of the water" had some kind of sexual connotation in 1920, when the book was first published.*
1920! Well before "that's what she said" became the standard response to any remotely ambiguous utterance, popularized in shows like The Office. I thus declare Freud the original promotor of the "that's what she said" joke. The drug-fueled old man may not have been the best psychologist, but at least in other incredible ways, he managed to worm his way into our lives.
(That's what she said.)
* I know I know, it's not the same type of joke. It makes more sense as written here, and is actually kinda the opposite of the current iteration. But written like that, with italics and all, as the punchline to a joke, the similarities are too awesome to deny. DENYYYHIII