To "nerd out" means to become very excited or enthusiastic about a particular topic, usually something that is considered geeky or uncool by mainstream society. People who are into comic books, video games, science fiction, and other such interests often use this phrase to describe their passion for these things. To "nerd out" means to lose oneself in the excitement of a particular subject, becoming so immersed that one may talk about it endlessly or engage in activities related to it with great enthusiasm.