If you have really bigguts you're a super bold and daring person. In fact, you might even be superman. That's how big your guts are. Don't worry, I don't tell anyone I know who you are. It'll be our little secret.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
The group is bold to commit crimes in broad daylight.