"Shizune" is a term from the adult visual novel game, "Katawa Shoujo." In this context, "Shizune" refers to one of the characters in the game. Specifically, she's the president of the student council at the school where the story takes place. She has a disability which affects her mobility but not her intelligence, making her highly respected and revered by her peers. The term might be used as a tag in porn videos to denote content that involves this character or scenarios related to her, including both sexual and non-sexual interactions