Where did the saying 'That's what she said' come from?

That's what she said...This phrase is a funny double entendre often used by Michael Scott (Steve Carrell) on NBC's comedy The Office. It is unknown who exactly coined the phrase. Many believe the first appearance of this joke in pop culture occurred in the 1992 movie Wayne's World.

While holding a Claudia Schiffer poster, Garth and Wayne say:

Garth: Hey, are you done yet? I'm getting tired of holding it.

Wayne: Yeah, that's what she said.

See the related link below for a YouTube clip of the quote.

In general "That's what she said" is a term used to indicate that someone has said something with a double, sexual meaning.

Example: I was at my grandma's and she was cooking something, and she had to go to the store, so she was teaching me how to cook what she was cooking. She said "When this light goes on, the oven is hot and you can stick it in" I replied with "Thats what she said!"