"fivehundo" is obviously an allusion to 500.
My first bike was a Kawasaki KX500...
Since I had obviously bitten off more than I could chew with that thing, my buddies used to tease me before weekends when we'd go riding.
They would ask "Are you gonna bust out your five-hundo?"
...the name stuck.
Here's a pic of me in between a double:
