This particular gesture suggests that your students are of West African ethnic descent. For example, if you come to meet any Mandinka language speakers and ask them to exhibit a ‘bisaro’ for you, you will then witness this same gesture. It is likely millennia old. Its meaning is generalized contempt for its target. In traditional West African societies it is common for men and women to ritualize certain separate expressive elements, the bisaro being one of them. I do not know exactly how this one came to reside primarily–though not exclusively–within the female province.