A pun is a play on words in which two words are used that have the same sound but have different meanings. Many different puns can be found all throughout Romeo and Juliet.
One example of a pun can be found in the very first scene. When Sampson declares, "Gregory, upon my word, we'll not carry coals," meaning, we will not be humiliated by the Montagues, both Gregory and Sampson then make plays on the word "colliers" (I.i.1-2). To be a "collier" is to be a person who either digs for or sells coals (eNotes). However, said with a British accent, collier sounds very much like the word choler or collar. So when Gregory replies, "No, for then we should be colliers," Sampson turns "collier" into "choler," meaning angry, as we see in the line, "I mean, an we be in choler, we'll draw" (3). Greggory next turns the word into collar, which refers to a hangman's noose. Since this play on words can only be heard rather than seen, as the words are spelled differently, it can be difficult for a novice reader to catch. But basically collier, choler, and collar all sound the same and refer to coal workers, anger, and the hangman's noose respectively, making all three a play on words.
A second pun can be found in the Nurse's lines when we first meet her and Juliet in Act 1, Scene 3. When Lady Capulet asks Nurse Juliet's age, saying that she is not yet fourteen, Nurse replies by making a pun out of the word teen in the lines:
I'll lay fourteen of my teeth--
And yet, to my teen be it spoken, I have but four--
She is not fourteen. (15-17)
The word teen means sorrow, but can also be interpreted to refer to a teenager. Therefore, what Nurse is saying here is that she would bet "fourteen of her teeth, but to her sorrow she only has four teeth," making a pun out of the word teen to refer to both sorrow and Juliet as a teenager.