To skirt is a verb, it means to straddle. It came about in the sixties when the freelovers would often straddle eachother in enclosed spaces while wearing skirts.
In movies and books, a skirt is a female who serves as a romantic or sexual object to a straight male, usually to the protagonist, member of an ensemble cast, or the audience.
A term peculiar to winos - a "skirt" is the brown paper bag which holds the bottle. Most often used when the fortified wine of choice is Wild Irish Rose.