“Between the Devil and the Deep”. In wooden ships, the "devil" was the longest seam of the ship. It ran from the bow to the stern. When at sea and the "devil" had to be caulked, the sailor sat in a bosun's chair (i.e. a short plank and a suitable set of ropes to suspend it from) to do so. He was suspended between the "devil" and the sea — the "deep — a very precarious position, especially when the ship was underway.