1. Someone who passes time in a languid, distracted, or aimless manner, half lost in contemplative thought but without a particular purpose; a day dreamer. I used to be worried that she was nothing more than a mooner with her head stuck in the clouds, but she proved to be one of the most intellectually advanced students this university has ever had. I spent a lot of my early teenage years as an angsty mooner, listening to melancholy indie music and musing over the nature of the world.

2. Someone who exposes their buttocks to others, typically as a prank (a practice called "mooning"). As the bus sped past us, a number of mooners stuck their rumps out the windows for us to see.