Sounds like a New York cuss word, doesn't it? Schlapen this, schlapen that, kiss my schlapen, etc.
Anyway, when a rooster is walking around, schlapen feathers are soft, webby feathers that hang on either side of the base of the tail plumes. When you buy a saddle skin, it's usually in the bag with the tips of the feathers at the top. If you spread the feathers at the "top" of the skin, the schlapen feathers will be in the back. They are great for wooly buggers, streamer hackle, etc.