She was gonna say 'Shitapan-kun'(Briefs boy), but she got it backwards and said 'kun shitapan' (Smelling undies)
Now how to translate the joke
No, what is backwards is what she is saying out loud and what she says internally. She was going to say "Stop calling me that" while thinking "what'choo say, shit-bread!?" but she got them swapped. It's a common joke done for character, where their spoken and internal dialogue are opposites or are expressing different view (typically their public view vs their actual view).
No, what is backwards is what she is saying out loud and what she says internally. She was going to say "Stop calling me that" while thinking "what'choo say, shit-bread!?" but she got them swapped.
Was the middle finger meant to be external or internal though?