When you're discussing
something geeky very intensely.
When you
know a lot of information about
something unusual, usually nerdy, and express it with great enthusiasm.
Bruce: Batman never killed his villains until
Robin died, which is the only reason why people began to call him 'The Dark Knight'. And in the series 'The Dark Knight Returns',
Robin was a girl after Jason died.
Wayne: Dude, relax. I only said I was excited for The Dark Knight.
Bruce: Sorry, I was nerding out for a minute there.