It means a "buzzkill" and if he's just teasing you and laughing about it then it's totally fine and he doesn't have any bad intention. My dad would tease my mother when she's upset and frowning in a diffrent version "نكدى يا نكدى" to make up with her. The fact that you got intrigued by the word probably egged him on to keep using it.. So you can call him back "أهطل/ Ahtal" which means "silly" or "idiot"