Under your belt

I've been learning to do a few new things lately. I keep hearing the prase "after you have a few thousand miles under your belt" (refering to motorcycles) and "after you have a year or two under your belt" (refering to programming). I'm curious as to what the origin of this prase would be. Thanks.

You are what you eat.

Do something to perfection and it becomes part of you.

To get something under (one's) belt is to get it into one's stomach.