A Capulet; a “fiery” young noble. (Shakespeare: Romeo and Juliet.) It is the name given to the cat in the story of Reynard the Fox. Hence Mercutio says, “Tybalt, you rat-catcher, will you walk?” (iii. 1); and again, when Tybalt asks, “What wouldst thou have with me?”
Mercutio answers, “Good king of cats! nothing but one of your nine lives” (iii. 1).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894