What does it mean to punk out?

"Punk out" is an informal expression that refers to an act of displaying cowardice, weakness or lack of courage in a situation where one is expected to be brave or tough. It is used as a derogatory term to describe someone who chickens out or fails to stand up for themselves when faced with a challenge or confrontation.

By saying this, the person wants to highlight that the other person's behaviour or actions were not brave or strong, but rather weak and disappointing.