Bold
Daring, Audacious
noun, な adjective
If you have really big guts 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.
だいたん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
グループは、大胆にも真っ昼間から犯行に及んだのです。
The group is bold to commit crimes in broad daylight.