No room to swing a cat
An awkwardly small, confined space.
Whether the 'cat' was a real moggy or the flail-like whip used to punish sailors in the British Navy isn't clear. Many reports claim that the cat in question is the 'cat o'nine tails'. As so often though, they don't supply evidence, just certainty. As a candidate for folk etymology goes the 'cat o' nine tails' story has it all - plausibility, a strong storyline and a nautical origin. That's enough to convince many people - the actual evidence shows the theory to be highly dubious. The phrase itself dates from at least the 17th century. Richard Kephale's Medela Pestilentiae, 1665:
"They had not space enough (according to the vulgar saying) to swing a Cat in."
The nature of that citation makes it clear that the phrase was already in use prior to it being committed to paper. The 'cat o' nine tails' isn't recorded until 1695 though, in William Congreve's Love for Love:
"If you should give such language at sea, you'd have a cat-o'-nine-tails laid cross your shoulders."
If those dates are in fact the earliest uses then the 'cat o' nine tails' theory is wrong. The task for anyone who wants to claim that theory correct is to pre-date those citations.
See also - nautical phrases.
See also - let the cat out of the bag.