What does the pun grave man mean

The men are serious and they are also doomed to an early death.

Mercutio is using a pun here, which is one of Shakespeare’s favorite literary devices. A pun is the use of a word that can mean two different things in the context in which it is used. Mercutio is making a pun with his tragic wound. During the fight between he and Tybalt, Romeo comes between the two of them and Mercutio is stabbed by Tybalt, which was not his intention. Tybalt is encouraged by his kinsmen to leave quickly and Mercutio goes into his final speech. Mercutio is dying and knows he will soon be in the "grave." He continues to make light of the situation and everyone feels he is just "clowning around" as always and they do not take him seriously. Even when he falls down the steps in the town square and is dead they think he is just fooling around till Romeo lifts the handkerchief and sees the wound in his heart.