I don't #
It means you're staring at something so stupid, you're too dumb to even be mad. It's like seeing a kid ride a goat to school.
"He told me he used a calculator to do 2 + 2. I said, 'I don't #.'""
"She said her dog wrote a book. I said, 'I don't #.'""
"He said he used a sword to fight a toaster. I said, 'I don't #.'"