Note to Romeo and Juliet, 1.4.40-43. "Tut, dun's the mouse ..."
40-43. Tut, dun's the mouse, the constable's own word:
If thou art Dun, we'll draw thee from the mire
Of this sir-reverence love, wherein thou stick'st
Up to the ears:
Mercutio is mocking Romeo, who has just said that he can't dance, can't enjoy himself, and is just "done." Mercutio takes Romeo's "done" to mean "dun." Dun, the color, is a kind of nondescript gray-brown, the color of a mouse, and somehow "dun's the mouse" came to mean "be as quiet as a mouse." This saying, "dun's the mouse," isaccording to Mercutiothe constable's motto ("own word") because constables were famous for sitting around silently and doing nothing. Mercutio is telling Romeo to shut up about being "done" and to quit being a do-nothing. He then adds that if Romeo is "done," he's Dun the horse, which was the name of a log that people pulled out of mud during a Christmas game. Only it's not mud that Mercutio and Benvolio will pull Romeo out of; it's "this sir-reverence love." "Sir-reverence" was short for "save your reverence," which was something you said when it would be offensive to use the word you really meant.
In short, Mercutio means that love is bullcrap, and that Romeo is stuck in it up to the ears.