Clabber is raw milk that's curdled: it's soured and thickened. It may sound gross, but clabber is edible.
This noun for naturally clotted milk has all but vanished from modern English — perhaps a victim of home refrigerators and supermarket shopping. It's the only English descendant of Irish claba, which means "thick." Clabber is certainly thick, and it's also quite chunky. The milk part — in the form of the Irish word bainne — was present in an earlier form of the term, bonnie-clabber. Clabber often consists of buttermilk, and it has frequently been a breakfast food.