I wrote a FAQ on this for .net many years ago (2005!). Here it is:
There have been various answers over the years.
Here's a few from the archives:
City Bike - quite a popular choice
Cool Bike
Chris' Bike
CB comes from the Japanese words "Cobasha - Biokeosan" meaning "Combustion Bicycle".
Also:
"K" stands for "Kairyo" or "Improvement". So, K1 would mean Improvement #1 (jonesdp)
"F" stands for "Four", or "Faster" (?)
I know for a fact that GXSR means "organ donor" - Evil Don...