An insult meaning that your sister is actually a transgender man. This insult is so rare it has only been said by 4 people and was buried in the Egyptian pyramids to never see the light of day.
Guy: ur mum gay You take out a golden box
Guy: Holy fuck hold on man I was just joking please don’t do this
You: Ur sister is a MISTER
World explodes
Grant: ur mum gay Mark: ur dad lesbian Grant: ur granny tranny Mark: oh yeah, well ur sister is a mister
Grant: *all veins inside body explode, has a seizure that causes him to pull out a gun from his jacket, and blow his own brains out.