“Taking a Johnny” isn’t your usual synonym for using the bathroom. It’s to be used on a much greater scale. “Taking a Johnny” must meet three criteria:
1. Must be at least 20 minutes in duration
2. Must be in a public bathroom
3. Must keep other waiting
Guy 1: "Where have you been? I've been waiting for over twenty minutes."
Guy 2: "I've been taking a johnny over at the gas station."