Mercutio makes one pun from the word "prick" in his lines,
If love be rough with you, be rough with love.
Prick love for pricking, and you beat love down. (I.iv.28-29)
The term "prick" can be used to mean "to pierce," or to "puncture," especially to cause pain (Random House Dictionary). However, since it can also refer to an "erected" object, like a dog's ear, Mercutio is also using the term with sexual connotations (Random House Dictionary).
Mercutio forms two other puns from the word "done." When Romeo continues to refuse to join his friends in crashing the Capulet's ball, saying, "The game was ne'er so fair, and I am done," meaning I am finished playing these silly games (40), Mercutio responds, making his first pun out of "done," with, "Tut, dun's the mouse, the constable's own word!" (41). While Romeo, in saying that he is "done," actually means finished, Mercutio makes a pun by rhyming the word "dun" to refer to the adjective "dun," which can describe a "gray-brown color," just like the color of a mouse. Historically, the expression, "dun's the mouse" meant "be as quiet as a mouse" ("Romeo and Juliet," shakespeare-navigators). Beyond that, Mercutio's phrase, "the constable's own word," also refers to the historically understood concept that the constable of a town "sat around" quietly "doing nothing." Hence, through the pun of "dun" referring to the silence of a mouse, just like the constable, Mercutio is chastising Romeo for sitting around and doing nothing ("Romeo and Juliet," shakespeare-navigators).
The second pun Mercutio makes by rhyming the word "done" with "dun" is in his line, "If thou art Dun, we'll draw thee from the mire" (42). In this line "Dun" refers to a historic game called "Dun the horse." The game was played at Christmas time and consisted of pulling a log out of mud ("Romeo and Juliet," shakespeare-navigators). Hence, Mercutio is saying that if Romeo is truly "done" then, due to his lovestruck behavior, he has also allowed himself to sink into "mire," or a poor state of mind. Mercutio is claiming that if Romeo is truly "done" then they will pull him out of the "dun," or mud, or mire.