Trashykawa is yet another nickname for the famous, grand/great king, by Iwaizumi Hajime. (Iwa-chan~~~~)
it’s most commonly used when bullying oikawa, and by the creator of the nickname himself.

(and if you didn’t know you the great king is it’s Oikawa Tōru)

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.