That's what she said

What is the origin and meaning of the phrase "That's what she said"?

If pronounced "That's what SHE said!" it is a means to turn a simple phrase into a dirty double entendre. An example given by the Wikipedia is:
'... if one were to say "It's too big to fit in my mouth" upon being served a large sandwich, someone else could say "That's what SHE said," [my emphasis] ...'

en.wikipedia.org/ wiki/Double_entendre

One may also find examples in the Urban Dictionary. Go to:

[Dead link removed - ed]

Plainly the sentence may also be used in the ordinary way. I don't know how far back the use as a dirty double entendre goes.
SS

As far as I know this saying is unknown in Rightpondia; over here its role has been filled by "as the bishop said to the actress" or "as the bishop said to the actress", as best suits the occasion. According to Wikipedia, it dates at least from WWII, possibly much earlier. en.wikipedia.org/ wiki/Said_the_actress_to_the_bishop
(VSD)

It's become newly popular as a catchphrase of Michael Scott, the office manager in the American version of The Office.