There are specific tutorials on how to craft a "parapet" (sill) from a single piece of sheet metal. Parapetówa Party ;) - LiveJournal
It can refer to a "piece" of a plant or a seedling grown in a "parapetowy ogródek" (windowsill garden).
A PVC or metal "piece" used to cover old windowsills without removing them. These are commonly found at retailers like Castorama .