What does "May one not speak" mean (the Nurse says it) Act 3, Scene 5 of Romeo and Juliet
In the aristocratic families of the Renaissance, many times an indigent relative, particularly a female relative who has not been married, would come and live with a family where she would be nurse to the children of her relatives. As such, the nurse was in integral part of the family and enjoyed many of the privileges of the rank of that family.
So, when Lord Capulet cuts her off, the Nurse feels that his action is a real effrontery. For, hitherto, she has participated in the raising of Juliet as a second mother, so to speak, and not as a servant to be shouted at. Secondly, the Nurse makes an effort to speak in order to intervene on Juliet's behalf since she is cognizant of Juliet's marriage to Romeo.
This line pretty much means just what it says. In modern English, the nurse would be saying something like "why don't I get to talk?" or "why can't I say anything." Just before this, Lord Capulet has pretty much told her to shut up and she wonders why she has to.
Lord Capulet wants her to shut up because she has been sticking up for Juliet. Capulet has been scolding Juliet harshly for not wanting to marry Paris. The Nurse stands up for her and Capulet gets mad at her.
This shows us that A) the nurse really loves Juliet and wants what's best for her and B) that she' used to being treated well -- she is surprised when she is told to shut up.