“There seems to be a whole plethora of reasons about how and why ‘chat’ has been associated in modern times with having a casual conversation with others. The term “Having a chat” is most commonly associated militarily with the first world war trenches and soldiers using lulls in the fighting to get together and de-louse each other while having casual conversations.
Chat is often ascribed to the Hindi word for a parasite, ‘chatt’ (with 2 Ts), but is more possibly from an earlier medieval English word for idle gossip, “chateren”. There are certainly references to it in 13th century english literature. But certainly Soldiers in the Napoleonic Wars referred to lice as ‘chats’…”