Short or passed best before date.
Milk sugar (lactose) can not be fermented by beer yeast and can therefore be used as a sweetener.
Commonly used in milk stouts.
Not as intensely sweet as regular sugar, so you need to add quite a bit for a noticable sweetness.
We recommend boiling it into a syrup before adding. Lactose can be quite difficult to dissolve otherwise.
Some types of wild yeast can ferment lactose.