History Lesson

While we may never know the exact origin of the phrase, one of the earliest, if not first, occurrences of 'That's what she said' in popular culture can be traced back to 1992 in the SNL sketch-turned-film, Wayne's World:

[Holding Claudia Schiffer picture]
Garth: Hey, are you done yet? I'm getting tired of holding it.
Wayne: Yeah, that's what she said.