Beleived to have originated from the word chodewagon (composite of Chode (or choad) and wagon)
"Wagon" is a shorter way of saying this but with extra insultation. It is safe to use infront of adults as wagon is a commonly used word, though with a different meaning.
Inspired by the Wagon Wheel adverts (ooh! you would), 'wagon' means an attractive member of the opposite sex. Often coupled with jokes like, 'I'd leap aboard that wagon and just ride it till the next town'. Cf 'bandwagon' - a GROUP of wagons.