I don't #
It means you're clueless. If someone is 'following' you in a convo, they get it. You? You're stuck like a deer in headlights. What they say is as clear as mud.
"I don't #" after he said he was going to rob a bank with a banana."
"She said she was going to explain the plot. I said, 'I don't #.'"
"He was telling a story about his dog. I said, 'I don't #.'"