What does it mean to misgender someone?

To misgender someone means to deliberately call someone a gender other than what they are presenting. It is usually meant in a disrespectful manner and is used to question another person's autonomy and right to define who they are to the world.

The term misgendering is usually used in the context of transsexualism, where someone supports a gender dichotomy and maintains that they were born with a body opposite of who they are. Some may use it in the context of transgenderism and other gender variance, but it makes little sense when used that way when TG persons don't always identify fully as a member of the gender opposite their body. The term is most useful and accurate when the person in question believes in a gender dichotomy.