This little-used piece of vacation terminology is a combination of the words "sand" and "silla" which is spanish for "chair."
When one would like to recline on the beach and there are no deckchairs available for you enjoyment, scooping sand out of a pit and into a tall pile will give you a an equally comfortable seat and backrest after less than one minute of excavation.
This method always uncovers wet, sub-surface sand, so it is necessary to sprinkle dry sand back on top to avoid any wetness if that bothers you. Perfect for reading a book.
"oh no, those money grubbing resort owners are charging 10 dollars to rent out lawn chairs and parasols."
"worry not, my equatorial amigo, a sansilla is economical, stylish, and environmentally friendly."
"cabron, ¿sabes que eres sumamente inteligente a veces?"
"thanks amigo. let´s get to digging."