No room to swing a cat

A popular and grim theory holds that "cat" here refers to the cat o' nine tails, the multi-tailed whip used for punishment in the Royal Navy. A full swing of the lash needed real clearance, hard to come by belowdecks. It's a plausible story given how many cramped-ship idioms exist, but there's no direct documentary link tying this specific phrase to that specific object; it may equally be a simple, literal image of a room too small to swing an actual cat by the tail (an old and unpleasant country pastime, sadly attested elsewhere in period writing).