To explain, without ever having felt the necessity to investigate the issue, that there are only two genders which are fixed and invariable for every individual at birth. Usually assumes equal lack of introspection on the part of transgender people who are likely to have spent their whole life questioning what gender means. The cisplainer is often shocked and angry when their cisplanation is not taken as absolute fact, criticized or even rejected altogether.
Some trans people also engage in cisplaining never having felt empowered to question the predominate cisgenderism of the society they were brought up in. This has been compared to a form Stockholm Syndrome.
An article is published in a newspaper describing how an individual, who was born male sex, has transitioned to a different gender. One of the first comments will cisplain:
"You can have as much surgery or medical treatment as you like but you'll always be a man (or woman)".
The possession of a platform to express visceral dislike/fear of a marginalised group or individual without being challenged
Germaine Greer has been denied her right to free speech by being denied a platform to express transphobia