Disrespectfully calling someone by their old name after they have changed their name. In particular, this is something done to transgender people.

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.

Deadname is a word used to describe someone’s old name after changing it (usually a trans person)

A deadname is the unused (birth)name of someone who changed their name, wether it has officially been changed or not. So technically, it's a dead name.

You could also call it a deadname because everyone who deadnames you shall become the dead /j

Referring to someone by the name they were born with instead of the name they chose for themselves after their mental breakdown.

Using a transgendered person pre-transition (birth) name or gender pronouns to refer to him or her.

The mere fact of calling a person by their old firstname when they've already legally changed names.