The term "yang-girl" seems to have its origin in Chinese culture, where "Yin and Yang" is a concept that represents the duality of life, with Yin representing femininity and Yang representing masculinity. In this context, a "yang-girl" refers to a female who possesses typically male traits or characteristics, such as being assertive, dominant, and confident.
In adult content, it might be used as a tag to describe a woman displaying these masculine qualities in sexual situations, either taking on a more active role or exhibiting behaviors that are considered traditionally "masculine." The term can also suggest a strong, independent and powerful female character, which may appeal to certain viewers who find such attributes attractive.