1. (informal) A dirty, grimy, sleazy, or disreputable person

- I'm just a teenage dirtbag, baby. Listen to Iron Maiden, baby...with me - Wheatus

2. (climbing) A poor climber, alpinist, skier or other outdoorsman who lives cheaply, without normal employment, and with few amenities in order to spend as much time on their sport as possible. Used praisingly.