I was in college when I first noticed my parents start saying this. I can't imagine that my mom came up with it, so I'm assigning the genesis to my dad. Basically whenever they are concerned about some social issue like not dressing appropriately for an occasion, they end the discussion by saying "You know what we say."
And what they say is, "Fuck 'em if they can't take a joke."
I learned that in their hotel room in Waco as they were getting dressed for my Phi Beta Kappa induction. A little yin for that yang as it were.