Macavity is a fictional character who is described in a poem in Old Possum's Book of Practical Cats, by T. S. Eliot. He also appears in Cats, the Andrew Lloyd Webber musical.
The name Macavity is a pun by T. S. Eliot, the author of the poem, on Macheath — a supervillain who appears both in John Gay's The Beggar's Opera, its sequel Polly and roughly 200 years later as Mack the Knife in The Threepenny Opera written by Bertolt Brecht and Kurt Weill in 1928, macuahuitl — the Aztec obsidian sword, and Moriarty — the surname of a supervillain-scientist from the Sherlock Holmes stories by Arthur Conan Doyle. Lastly, the word 'cavity' implies a hole or void or absence of something, and he is described in the poem as being "not there" at the time or location of any crime.
The poem Macavity the Mystery Cat is the best known of Eliot's Old Possum's Book of Practical Cats, the only book Eliot wrote for a younger audience. The poem is considered particularly suitable reading for 11- and 12-year-olds.
Macavity (also called the Mystery Cat, the Hidden Paw and Napoleon of Crime) is a master criminal, but in the poem he is too clever to leave any evidence of his guilt. There is a resemblance with Professor James Moriarty from the Sherlock Holmes stories by Arthur Conan Doyle. In a letter to Frank Morley, Eliot wrote, "I have done a new cat modeled on the late Professor Moriarty, but he doesn't seem very popular; too sophisticated perhaps." Sherlock Holmes describes Moriarty as "the Napoleon of Crime" in The Adventure of the Final Problem and a "Napoleon gone wrong" in The Valley of Fear. Evidence that Macavity was based on Moriarty was first presented by HT Webster and HW Starr (Macavity: An Attempt to Unravel His Mystery, 1954), and later rediscovered by Katharine Loesch.