Came across this one a few times (most recently here) where it links to a number of mountain elements:
you still end up with some neat mini-BURPS and other useful pieces
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I've heard variations on "Big Ugly Rock PieceS".
However, I've also seen the term used to describe any large piece that the builder thinks has little reusable value due to its shape being pretty specific to a certain use. Obviously the mountain pieces are built to look like pieces of rock. But there's also some tower elements in the Harry Potter sets that often fit description.