Nicki Minaj was the one who came up with this statement. Basically when somebody says something irrelevant or weird you can use this to pretend you didn't see it.

n. The birth name of somebody who has changed their name. Most commonly attributed to trans people, but can be attributed to any person who has changed their name. (sometimes written as two words: dead name)

v. 1. To call somebody by their deadname.
v. 2. To out somebody's deadname to the public.