"That's what she said" is said after a double entendre. (a figure of speech in which a spoken phrase is devised to be understood in either of two ways. Often the first meaning is straightforward, while the second meaning is less so: often risqué, inappropriate, or ironic). Example: Person 1: "This is so hard." Person 2: "That's what she said!" Person 1 could have been referring to a desk or part of the male anatomy. :P Michael Scott, on the show The Office says "That's what she said" often. (I apologize if there is already a group for this. I looked but I didn't see it. Hopefully, I didn't miss it.)