Kind of a dejavu moment. Feel like I've asked this question before. Hmmm
Since I was the first to use the term in this thread...I tried to remember why I call it that but it's just what I call the oil tank on a harley, like I call the cylinders jugs. I didn't make it up but I sure can't recall where I heard it except I know it's been many years.
Maybe it has something to do when the tank moved out of the gas tank back with the knuckleheads? The horseshoe tank is round more like a bag...perhaps. Just guessing.
Harley has odd terms for those "in the know". I do know I used to call it a kick stand until I got my first harley and I was corrected by an older rider who told me I have a jiffy stand now. But I called it an oil bag even when I called it a kick stand.
I even googled it but seems the question comes up from time to time but the answer always seems to be "that's just what it's been called a long time".