It's likely a made-up word, perhaps based on the word "banter," which means:
* Light, playful conversation or teasing
If someone uses "banterous" they are probably trying to describe something that is full of playful teasing or lighthearted conversation.
For example, they might say: "The atmosphere at the party was very banterous," meaning there was a lot of lighthearted joking and friendly conversation.
However, it's worth noting that using "banterous" is unusual and might not be understood by everyone.