When two people who love each other very much have strange sex games, one may lick the other's bumhole with quite some vigour. If they emerge with faeces spread around their mouth, then they have a "chocolate muzzle".
If you are the person who is being licked, you can be said to have "given them a chocolate muzzle", and the rubbing of poo around another's mouth, even forcibly, is also known as "giving them a chocolate muzzle".
It can be used as such:
"Eustace, if you do not come back here at once, i may have to give you a chocolate muzzle!"