Nay, as they dare. I will bite my thumb at them, which is a disgrace to them, if they bear it. (bites his thumb) (1.1.36)

Biting your thumb—placing a thumb behind your front top teeth and then flicking it out—is a symbolic gesture similar to “flipping someone off.” The action is a silent and immature way to insult someone and could be interpreted as an invitation to violence. In this quote, Shakespeare reveals that the young men involved in the feud between the Capulets and the Montagues, Sampson in particular, are immature and “looking for a fight.”

ABRAM: Do you bite your thumb at us, sir?
SAMPSON: (aside to GREGORY) Is the law of our side if I say “ay”?
GREGORY: (aside to SAMPSON) No.
SAMPSON: No, sir. I do not bite my thumb at you, sir, but I bite my thumb, sir. (1.1.39–42)

Abram understands the symbolic meaning behind Sampson’s biting his thumb and takes offense at the gesture. But Sampson, once he realizes he may be in legal trouble if he admits to the insult—for such an action would constitute inciting a fight—cowardly denies he was biting his thumb at Abram. Such a scene reveals the false bravado of these foolish young men.

SAMPSON: But if you do, sir, I am for you. I serve as good a man as you.
ABRAM: No better.
SAMPSON: Well, sir.
GREGORY (aside to SAMPSON): Say “better.” Here comes one of my master’s kinsmen.
SAMPSON: (to ABRAM) Yes, better, sir.
ABRAM: You lie.
SAMPSON: Draw, if you be men. (1.1.47–53)

At first, Sampson agrees that his master is no better than Abram’s master. But once Gregory and Sampson know they will outnumber the Capulets’ men, they collude to goad Abram into a fight by having Sampson change his tune and say his master is better. This juvenile taunt is similar to Sampson’s biting his thumb at Abram earlier and shows how immature and foolish the Capulet–Montague feud is.