During World War One the entrenched soldiers, the off-duty ones, would pick lice off their clothing and flick them into the candlelight to kill them. Being that a nickname for a louse is a ‘chat’, the act was known as ‘chatting’ (the term in this sense had already been around for many years, maybe since the 16th century). As this delousing would take some time the soldiers would talk amongst themselves. The term ‘chatting’ eventually became attached to the act of having a conversation, and hence the name was born.
(There are other stories that suggest that it’s a shortened version of ‘chew the fat’, or ‘chatter’, or that it’s a derivation from the social drug ‘Khat’. But I learnt this original story when I was a teenager, and although the name may have originated from a different source, I think that many Brits would have picked it up from returning soldiers (and their reasoning for the name). As although it was more than likely used prior to the war, it’s common usage has occurred since.)
[carried over from ‘anyadditionalinformation’ wordpress blog]