They were happily chinwagging about the latest gossip in the coffee shop.

chinwagging 是动词 chinwag 的现在分词形式。chinwag 是由 chin（下巴）和 wag（摆动）组成的复合动词，形象地描述人们聊天时下巴摆动的样子，意思是"闲聊、闲谈"。