When a cis person explains
something to a trans person that they already know or explains trans issues to a trans person, when they (the cis person) themselves
don't know what they're talking about, because well they're cis and don't
experience trans issues.
"I'm not transphobic, transphobia is the fear of trans people, and being trans gender is due to a mental disorder called
gender dysphoria"
"You're taking the word transphobic
out of context, being trans isn't a mental illness and the treatment for gender dysphoria is
transitioning, you're such a cis-splainer, stop cis-splaning"