When a trans-person tries to explain your sexual identity or orientation to you contrary to the fact that this is a personal choice and everyone has the right to identify as they please.
Person A: I'm super-straight
Person B (trans): That's transphobic, genital preference isn't a sexual identity..
Person A: Please don't transplain to me, I know what I am and it's not for you to decide. Transplaining is superphobic.
When a Transgender person or an ally tries explaining sexuality and science to Super Straight people and tries to invalidate their very real and valid identity in the process.
Trans: there's no such thing as a biological female read up on your science. And Super Straight can't be a sexuality it's a preference.
Super Straight: You're Transplaining. I don't know where you're going with this... But it's not in Super Straight pants.
When a transgender person (generally who was previously male and has transitioned or is in the process of transitioning) continues to exhibit "mansplaining" behaviour, but not longer associates with that gender.
"OMG I was just showing them my makeup routine, and they started transplaining to me about why I shouldn't be using that foundation for my skin."
"Calorie deficits don't work. I read a study on Huffington Post about metabolism, you need to do your research, let me explain the science you, it's why I weigh 300 lbs and it has nothing to do with my lifestyle choices."
-Fatsplaining