IDK if chattering is a house cat thing or not but I’m just gonna assume it is (and, therefore, it is for this kitty world), since feral cats don’t really make a whole lot of noise.
So just imagine Firestar and Graystripe laying around Sunningrocks (y’know, between fights for it) in the middle of Greenleaf, and this sparrow happens to fly by. Firestar lifts his head off his paws, mildly amused (they’re all well-fed and it’s too high to catch anyway) and starts chattering at it without even thinking about it.
Graystripe immediately sits up. Bro, bro. Bro, what in StarClan was that?
Firestar stops because AW CRAP THIS IS A KITTYPET THING ISN’T IT and gets all worried that cats are gonna start whispering about his blood and whether he’s a true leader again but Graystripe is like nah, dude, just tell me how to do that. That noise was so weird, I like it.
So Graystripe becomes the first ThunderClan-blood warrior to chatter, and he’s, like, so into it. He thinks it’s the silliest noise a cat could make and he wants everybody to hear it. The kits laugh and join in- the apprentices do it as a joke but it ends up sticking. The elders scoff and put their heads on their paws, ultimately giving it a try when some uppity apprentice dares them to try. Firestar is walking around camp one day and almost trips over his fluffy ginger paws when he hears Sandstorm- his serious, no-nonsense Sandstorm- chattering at Leafkit and Squirrelkit. She tells him it’s something she imagined the kits would have a lot of fun with.
The other Clans are baffled and oddly amused as chattering sweeps over the Gathering every full moon. The odd warrior will latch onto it, but mostly it’s just a ThunderClan thing.
And that’s how ThunderClan got the name for being a Chattering Clan.