A Nut Mullet is when a man shaves exclusively the visible part of their scrotum. In doing this they leave the back and bottom of the ball sack leaving what can be seen as a mullet for your nuts. This is likely to be seen on the sack of a married man, a man in a committed relationship or a man who's given up all hope on life.
Amy's chin was gently ticked by the Nut Mullet of the man she was sucking off.