X's - Honda VTX owners refer to their rides as X's. (thanks to Eric Inger)
Xylophone - A musical instrument (please do not play the xylophone while riding, not only is it dangerous, but highly annoying).
Yahamahog - A Yamaha that some one made to look like a Harley chopper (raked, big front wheel etc.)
Yamadog - Name for a Yamahahog rider
Yamahaharley - Japanese bikes that look like Harleys (eg. Kawasaki Vulcan Classic)
Yamahammer - Yamaha
Yamahauler - A truck or trailer that is hauling a Yamaha
Yard Shark - Dogs that come out of nowhere and try to bite your tires. Caution: Can cause motorcycle crashes, handle this type of situation with care.
Z Bar - A handlebar that sweeps out of the risers toward the front of the bike and then sweeps back again towards the rider. A popular handlebar from the 1970's.
Zook - Suzuki
Zorst - Exhaust (motorcycle pipes)