https://books.google.com/books?id=Dx0GAAAAQAAJ&pg=PA204 Letter X, page 204 At length I came within sight of them, three in number, where they sat cosily niched, into what you might call a bunker, a little sand-pit, dry and snug, and surrounded by its banks, and a screen of whins in full bloom. […] https://books.google.com/books?id=Dx0GAAAAQAAJ&pg=PA223 Letter XI, page 223 And are ye in the wont of drawing up wi' all the gangrel bodies that ye meet on the high road, or find cowering in a sand-bunker upon the links?

Used to refer to a sand-filled hollow in general.