A pocket or fold of skin on a baby's neck, under the chin, that collects dribbled milk from the bottle or nipple, and remains trapped. The milk begins to go rotten and cause your child to stink horribly. When you give the child a bath or shower, you will notice brown sludge hidden away in the milkflap.
Husband: I am giving the baby a bath OK?
Wife: OK, but make sure you wash that stinking milkflap, it's getting pretty rank!