in the US, pants are trousers, so the etymology of "underpants" is sound... "pants" came from "pantaloons" in the 18th century, but it was then shortened to "pants" and the term "pantaloons" was given to describe women's underwear. the original pantaloons were also worn in america, but were called "knickerbockers" because of this guy: diedrich knickerbocker. you'd think that an abbreviation of "knickerbocker" and a slip of the tongue would give rise (ahem) to "boxers", but that term was adopted because of the loose-fitting shorts (see below) worn by actual fighting boxers. the "knickerbocker" was abbreviated to "knickers" however - as the pantaloons were worn under hooped skirts by the continental women, so were knickerbockers worn by their US counterparts. trousers are a different animal, the word "trouser" coming from the french "trouse" which, in turn, came from the gaelic/middle irish "triubhas" meaning close fitting shorts (see above), and they obviously grew from there. "slacks" come from the nautical reference to slack sails, and refer to the looser-fitting trousers worn by the american military. "shorts" comes from an abbreviation of the term "short trousers". "pantyhose" comes from the german for trouser "hose" and refers to a tight covering for the leg, giving us "tights", later called "nylons" because of the invention of nylon (a synthetic fibre made by the meniscus between two liquid polymers) by someone in New York and someone else in LONdon (see where they got the same?). notice the paradox with trousers and shorts - one comes from the other comes from the other... ah well.
and I had to whisper "they mean vacation" 
