The term “You don’t say” started as a rage comic face (or otherwise known as “rageface”) based on a contour drawing of Nicolas Cage, which is used as a sarcastic response to obvious statements or observations.
The term and the meme it has spawned is used in very similar way like the ORLY images, which feature owls.
The original image for “you don’t say” was taken from the 1988 black comedy movie called Vampire’s Kiss, where Nicolas Cage is playing as the character Peter Loew, who is literary agent, whose madness is convincing himself that he’s turning into a vampire.
The image is taken from the scene where he is speaking to his secretary in a disturbing unique manner.
Later this image was used as the term/meme “batshitcrazy” which was turned into the Nicholas Cage meme “you don’t say” we all know today in 2011. In a Reddit post.
The term/meme “You don’t say” started to get popular right after the reddit rage comics post which has prompted it to spread all around the internet as a sarcastic meme.
It had a huge jump in popularity thanks to social media sites like Facebook in 2012 because people started to use it to sarcastically criticize the dumb posts made by some users.