This term refers to a man "gaining" an inch to his dick by having done something extremely badass/manly, thus becoming even more manly. Men can wear these accomplishments as a medal and brag about them at later dates.
Dude #1: "Man yesterday was crazy! I definitely gained an inch."
Dude #2: "What did you do to earn that accomplishment?"
Dude #1: "I grew a beard, threw on my plaid shirt and bibs, and went out to the forest and chopped down trees to make myself a log cabin."
Dude #2: "Damn!"
The most nerve wracking awkward shiz of your whole life. You must shoulder check every two seconds and have both hands on the wheel, while a smelly person with a clip board critiques your driving in an awkward silence. But sometimes they ask you awkward questions like "So where do you work, or are you a student". to which you reply with a lie because you can't say that you are a student majoring in drinking and ton and masturbation.
Man I have my driving test tomorrow
Whatever you do, do not say fuck, or balls, or make frog noises to break the awkward silence