Douglas Harper's Etymology Dictionary
Wiktionary
Describing a voyeuristic image of the view up a woman's skirt. n. An image of this kind. v
(context transitive English) To take a voyeuristic photograph up the skirt of (a woman), especially surreptitiously.
Wikipedia
Upskirt refers to the practice of making unauthorized photographs under a woman's skirt, capturing an image of her crotch area, underwear and sometimes genitalia. The term "upskirt" can also refer to a photograph, video, or illustration which incorporates an upskirt image. The term is also sometimes used to refer generically to any voyeur photography – that is, catching an image of somebody unaware in a private moment.
The practice is regarded as a form of sexual fetishism or voyeurism and is similar in nature to downblouse. The ethical and legal issue relating to upskirt and downblouse photography is one of a reasonable expectation of privacy, even in a public place.