Concealed way of saying phuct. Both ways of pronunciation have been in use since the 1980s. I first heard the expressions used in Lancaster University.

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.