It's like a cis person, but they identify with their gender even more than 100%. A supergender male, for example, might feel that he is 500% male and inject himself with a bunch of shit that'll probably not be very good for him, just to reach a supreme degree of masculinity that no standard cis man could ever reach! Supergender people are valid too! Some synonyms for the term include: gigagender, ultragender, hypergender, and megagender.
It's definitely one of his worst books otherwise but this (and other explorations of voluntary gender modification) was a concept in Distress by Greg Egan.