No, the insult wasn’t delivered in Taiwanese. It was delivered in full, clear Mandarin. The only part I lacked, initially, was the frame of reference necessary to fully understand the exact nature and full degree of the insult. I am aware of most common Taiwanese insults, along with a few I also got from students that I’m sure would similarly stump most readers here as well.
Not surprising, really. Kids in every corner of the world have their own “student language,” often unintelligible to those not in touch with youths directly. I think this term is very situational. It doesn’t have its connotative meaning when said out of context. I think if your friends heard it in context, they’d get the general gist of the meaning. When I heard it, I knew it was an insult, just not exactly in what way, simply because of the timing and delivery of the term.