Underswap Gaster and Kaithlyn dynamic headcanons (gaster gang ver.)
Lyn would curse a whole lot in the house along with Fell. When Swap caught on and told them to stop cursing, Lyn acted all surprised and dramatically said, “Ooh no! How will I curse now~” But unlike Fell, she could just switch languages. She would just switch to Hungarian. Swap couldn’t understand it anyway, or at least that’s what she thought.
One time, while she was making a quick snack for herself in the kitchen, she cut herself with a knife. “Kurva anyád!” she hissed in a harsh, whispered tone, before Swap appeared behind her and took the knife away. He held her hand and used his healing magic on it. “No swearing,” he said with his usual smile, focused on her hand, while Lyn just stared in disbelief. He didn’t… no, he wouldn’t have known. Maybe he just knew when she was cursing based on the tone of her voice.
Later, after the other Gasters were fighting anomalies, Lyn got herself hurt. She yelped as she fell onto the cold ground, void, or whatever that black ground was. “Picsába!” she yelled, holding the arm that had been lashed at her. Suddenly, a loud scolding yell came from not far away, where Swap was fighting a different group of his counterparts. “LYN MIT MONDTAM A KÁROMKODÁSRÓL?”
Swap had basically learned Hungarian curse words and even some sentences just to scold Lyn if she cursed in that language again. He learned it just to tell her not to curse. Swap takes his no-cursing rule very seriously.
Now Lyn has to think of new ways to curse without Swap catching on. She wonders if she were to learn curse words in a different language, would Swap learn that language too just to scold her again?
Spoiler alert: yes, he would.