Cat o' nine tails \Cat" o' nine" tails`\, cat-o'-nine-tails \cat"-o'-nine"-tails`\n. 1. a whip used as an instrument of punishment consisting of nine pieces of knotted line or cord fastened to a handle; -- formerly used to flog offenders on the bare back; -- called also the {cat}. It was used in the British Navy to maintain discipline on board sailing ships. Syn: cat. [WordNet 1.5 +PJC]