Prefixes are usually added to the beginning of words to change their meaning. "Skirt" is already a complete word on its own.
Perhaps you're thinking of a word that *includes* "skirt" as part of it?
For example:
* Underskirt (a layer worn underneath a skirt)
* Mini-skirt (a short skirt)
* Tutu-skirt (a type of skirt made of tulle)