Nickname for the poor sod who’s turned up on their own, looks left out, or never gets invited. Billy is just a generic bloke’s name, so it’s basically saying you’ve got no mates. Usually it’s harmless UK ribbing between friends, but lob it at the wrong person and it’s a proper insult. Works in playgrounds, pubs, and group chats alike.
"Jay turns up to the pub an hour early and parks himself in the corner. Oi, Billy No-Mates, you been dumped or you just can’t read the group chat?"