The term "hard-nips" is a colloquial way to describe a particular fetish or preference in adult content. In this case, the focus is on the female nipples being depicted or acted upon in an explicit manner. These videos typically include scenes where the nipples are hard or erect, often due to sexual arousal or manipulation. The term is used by adult content creators and viewers alike as a descriptive tag for searching and categorizing content that features this particular theme.