The cat o' nine tails is a type of multi-tailed whipping device that originated as an implement for severe physical punishment, notably in the British Royal Navy.

The word is recorded in English since 1695, and it was probably so called in reference to its claws, which inflict parallel wounds, similar to that of a cat’s claw.

There are equivalent terms in many languages, usually strictly translating, and also some analogous terms referring to a similar instrument's number of tails (cord or leather), such as the Dutch zevenstaart, seven tail.

The instrument traditionally has nine thongs as a result of the manner in which rope is braided. Thinner rope is made from three strands of yarn braided together, and thicker rope from three strands of thinner rope braided together. To make a cat o' nine tails, a rope is simply unraveled into three small ropes, and each of those next unraveled, again in three. A rationalization, probably conceived after the fact, for the number nine is that nine is thrice three, a Trinity of Trinities, fitting the concept of the wrongdoer going against the God of the Anglican or Catholic Church and hence against the Holy Father which, theoretically, thus puts the wrongdoer back on the path toward righteousness. It is also said that sailors had a holy cross tattooed on their backs to prevent it from the unreligious act of being flogged, but there is no evidence for naval authorities awarding such exemption.
The naval "cat", also known as the captain's daughter, since, in principle, it was only used under his authority, weighed about 13 ounces and was composed of a baton and nine cords.

All formal punishments, ordered by captain or court martial, were given ceremoniously on deck, the crew being summoned to witness punishment and drama enhanced by drum roll and a whole routine, including pauses, untangling of the tails, a drink of water and so on which is believed were more intended for the observing crew than for the actual participants.