This refers to the bottom edge of the mainsail, known as the foot. If this was not securely tied to the boom and became detached, it was said to be footloose. In naval terms, this means stop talking and be quiet. This was the last signal from the bosun’s pipe, which meant lights out and shut up. In shallow water, the keel of the ship might touch the bottom but not get stuck and sail on. As punishment, sailors were flogged by the bosun’s mate, using his cat-o’-nine tails.