What are maidenheads in Shakespeare's play Romeo and Juliet?It was in the conversation between Sampson and Gregory in Act 1 scene 1.

sam. 'tis all one I will show myself a tyrant: when I have fought with the men, I will be cruel with the maids; I will caught off their heads.

gre. The heads of the maids?

sam. Ay, the heads of the maids, or their maidenheads; take it in what sense tou whilt.

gre. They must take it in sense that feel it.

In this scene the humor is derived from the wordplay of a maiden's head (literally the heads of Montague women) and the maidenheads of the Montague girls (ie their hymens). This might seem gruesome but whole scene is humorous banter.

gre. To move is to stir, and to be valiant is to stand; therefore, if thou art moved thou runnest away.

sam. A dog of that house shall move me to stand: and I will take the wall of any man or maid of Montague's.

gre.That show's thee a weak slave for the weakest goes to the wall.

The scene even begins with a hotheaded Sampson stating he and Gregory will not be insulted  as low or base "we'll not carry coals". Gregory who seems calmer and good humored responds jokingly, taking Sampson's comment literally. Sampson then explains he means if they are insulted (an we be in choler=angry), they will fight (we'll draw...our swords). Gregory replies with the advice "ay while you live, draw (wordplay on drawing out a sword) your neck out o' the collar (hangman's noose), insinuating that if they fight, they could be hanged. Sampson says then, he'll strike quickly, being moved (being provoked). Gregory suggests that Sampson is a coward "thou art not quickly moved to strike" which is prooved in the next scene

sam. My naked weapon is out. Quarrel, I will back thee. (I've my sword out. You go fight, I'm right behind you)

The wall which they talk about is the highroad. Back in the day, the streets were really nasty so the nobility and women were give the part of the walk closer to the houses (the wall) whereas the servants or men took the side closer to the road. Therefore "I will take the wall of any man or maid of Montague's " means I am better than anyone in Montague's household.  Gregory retorts that the weakest go to the wall because women, old people, ect. are offered the wall.  To end an explanation of the scene:

sam. they shall feel while I (my penis) is able to stand; and 'tis known I am a pretty piece of flesh( well hung)  Gregory responds it is good that you are not a fish, for if you were you would be the shriveled poor John fish (you are not well-hung). Hey get your sword out the Montagues are coming.

The whole scene is just a comedic way to open the play. It is really just a fancy locker-room conversation between the 2 servants . hope this helped clear it up.