I only recently heard this punchline. My wife says it goes back a long, long way, maybe even before our childhood. What’s the earliest you remember hearing/saying this?
From at least 1975, and in a factory environment. It was humor most high. One just smiled and nodded.
Before you were born.
I have never heard this. Can you provide a context?
It is a usually male-specific (in that case ostensibly heterosexual) response, and the result is intended to be ribald. As in, “I dunno if I can take any more of this.” “That’s what she said.”
You want to put the emphasis on “she”. Some kind of hilarious, eh?
Yeah. Maybe if I watch mute’s youtube Office clip 20 more times I’ll start to get it.
Who’s on first? 
With best regards.
Pfreddee
That’s what she said.
That’s what she said.
atta boy, don’t be afraid of takin’ the cheap shots 
Thank you folks. I’m here all week. Don’t forget to tip your waiters.
That doesn’t work, because they’re not like cows. They usually try to get back up again.
Hu is the president of China.
Best wishes,
Jerry
Over here in English-speaking Yurp, the phrase used to be “…As the actress said to the bishop.”
This dates from time immoral.
Used to be??? Still in common usage isn’t it? 
This dates from time immoral
Love it.
(That’s what SHE said.)
It’s being edged out… I blames the internetwebs.
In that case I shall be a reactionary old git and start campaigning for the return of ‘As the actress said…’ with immediate effect.
I prefer the actress talking to the bishop because it adds an extra level of humour, the chance that a ‘holier than thou’ Bishop might be getting up to no good…
now where have I heard that recently?