Trap

"Trap" is a slur against trans women
Pearl chalkboard edit originally from Twitter, by Anna Harren.

This is the reason the word trap causes such controversy. While in the context of some anime referring to certain characters who use feminine dress to trick people it might be appropriate. Within the anime fandom a problem is fans who insist that all characters regardless of how they behave or what they say are simply traps and cannot be transgender. This reflects the mindset out in the world that real life transwomen contend with being thought of as simply out to "fool" people.