I’m fairly certain it’s just a cutesy nickname, no extra meaning attached.
Bakugou is his last name–his full name is Katsuki Bakugou (most people address each other by their last names though, hence most people calling him Bakugou-kun.) And “-chan” is a common honorific for girls and for little kids. My assumption is that when Bakugou was much much younger, friends and teachers called him Katsuki-chan, which then got shortened to Kacchan.
I’m not overly familiar with the nuance of honorifics, but this cracks me up because it’s like three layers deep of “cutesy nickname”-ness
- It uses Bakugou’s first name, which is already unusual
- It uses the -chan honorific, which is NOT typical for a 15 year old boy
- It’s even cuter because it’s a petname version of Katsuki-chan (Kacchan)
This just cracks me up endlessly because it’s being used for fucking Bakugou of all people. The angriest, most violent 15 year old in the world. Is fucking “Kachaaann~~~” And what’s more it’s Deku of all people who he lets call him this.
It’s so stupid and counter-intuitive I love it.