An old phrase which reentered popular culture in the early 90s after being used in The Simpsons by the character Ned Flanders. It subsequently became a minor catch phrase of his, used in some Simpsons merchandise.
It means exactly what it says (the "say we did" part may or may not be sarcastic) but is often said in a patronizing tone or otherwise as a witty remark in a variety of situations.
Girlfriend: Hey we should go ballroom dancing sometime it looks like fun!
Bloke: Uh let's not and say we did...
Pissed Dude (Getting Carried Away): Huh that 60 year old barmaid looks like such a slut. I bet you could pull her. Infact I bet we could be fucking her from both ends at the same time before the night's out haha, that's how much of a desperate slut she is.
Other Guy: Haha yea but let's not and say we did!
Pissed Dude: Hah damn right