This expression has existed since at least the 1600s and means that someone is in a predicament or a dangerous place with no easy way out. This expression believed to have its source in the historical nautical practice of sealing the seams between a ship’s wooden planks with hot tar. The devil is the name possibly given to the garboard seam, the ship’s longest seam, which is typically the most prone to leaking. When a sailor attempted to caulk this seam by hanging off the side of the vessel, he was in serious danger of failing overboard and drowning,. If he didn’t caulk the seam, the ship could fill with water and sink. In other words, the sailor was faced with two difficult choices for a sailor: risk his life to repair the ship or risk the entire ship by not repairing it, thus caught between the devil and the deep blue sea.