B-string - A B-string is the type of underwear you wear when you don’t

A B-string is the type of underwear you wear when you don’t care if your pants fall down or if people see your business.

I wore my B-string to the gym and my trainer asked if I was doing a new workout.

My B-string is so ratty it looks like it was run over by a truck.

I wore my B-string to a party and my friend asked if I was dressed for a zombie apocalypse.