Recycling the usage of your dirty undergarment by flipping it inside-out on it's "side B" which is supposedly cleaner than it's "side A". People who're lazy to do their laundry do this all the time.
"Dude isn't that the same Calvin Klein undies you were wearing yesterday when we went to the beach?"
"Don't worry man, it's on it's Side B"
"That's just gross dude..."
Someone who's a loser and/or an asshole.
Dude, you're such a side B.