Enforcer of mothers' wills. Doesn't need to be an actual belt, just the notion. Sometimes the enforcer of fathers' wills, also.
Boy 1: Man, I'm in advanced classes.
Boy 2: Why?
Boy 1: Yeah, cuz my mom said I had to.
Boy 2: Do you always do what yo momma tell you to?
Boy 1: Yeah, she got a belt!