An uneducated and improper way of gaining mass. This is done by eating extremely high fat and low protein foods (McDonald's, Ho-Hos, etc..) with little if any weight lifting. The would-be jabroni does gain mass - Yes, but it is almost entirely fat, defeats the purpose and is next to impossible to cut.
Ian: "Hey have you seen Leroy lately? He's been pounding those Twinkies and Big Macs. He said he's bulking up and trying to get diesel, but all he did was end up a fat ass."
Mike: "Yeah, I know. Dude needs to pick up an MD, you know - a Muscular Development (magazine)... He's such a dirty bulker. So un-diesel."