A phrase used in a mocking sense on 4chan, often as a reply to
people to discredit their opinions due to them being transgender.
Due the anonymity on the site, every post looks the same, meaning
people can imitate each other at
will. So someone who might want to make fun of another person would say this as a reply, imitating an edit or additional note by the original poster.
"I think that the Elden Ring
game is good."
"I'
m trans
btw, not sure if that matters." (<-- not the original poster)