orlop (AWR-lop) - n., (Naut.) (of a wooden ship with four or more decks) the lowest deck, above the hold.
Which is usually the fourth deck down from the main deck -- any more decks, and the ship gets top-heavy. The orlop was usually where the cables were stored -- it wasn't a good place for crew quarters as it was below the water line, and definitely couldn't hold canon because same.
Orlop deck shown in red (thanks Wikimedia~!)
Origin? Well, like a lot of nautical terms, we're not really sure: it could be from the "overlooping" of the cables, or an alternate form of "overlap" as in the boards covering the hold below it, or from Dutch/Middle Low German overloop/overlōp, a running over/extend. Your guess is as good as mine, here.
Still he prowled about the after orlop deck, and talked at large of his anxiety for the contents of the bullion-room.