It is said with a long “R”. Like when you are agreeing or “backing” someone’s just said statement or comment; or to “second” the notion, if you will. To sum it up. It is commonly used in the book of Ebonics, street slang style type talk.

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.