Don't ask me why I know this, but John Mayer is pontificating on the meaning of douchiness. He should have just checked Urban Dictionary. Or he could swing by here for a visual explanation (thanks to hotchickswithdouchebags.com for the pics that obviously came from that site):
This guy is a douchebag.
Lest there be any confusion, here is a handy chart to help you spot a douchebag (click on the picture for a larger image):