And if the new usage becomes popular, it ends up in the dictionary, which is what happened here.
She said it wasn't a word. If it's in the dictionary it's a fucking word. Not only was she a douche to correct him, but she was also wrong.
I understand that snuck is not a word often used in American literature but she's having a conversation. Fucking douche trying to look smart. Ordinarily I would say it was wrong for Conan to punk her like that but in this case it was well deserved.