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".