Example:
After some hours of working overtime, you get to your friends on a party. They already had several drinks and thanks to their blood alcohol level, they're having a lot of fun, while you're still sober and therefore not in the same mood as them. To change that, you've got to level your own blood alcohol concentration up.
If people eventually stare at you because you drink a lot in a very short amount of time, you are allowed to excuse this by saying: "It's alright. I'm just leveling up right now."
A colloquialism for masturbation, orginated from the "World of Warcraft" paladin forums when a certain player wrote an essay on the paladins boring gameplay. He pointed out the positve side of only needing to attack a monster once every few minutes, then leaving yourself on auto attack while you "download gigabytes of hot girl on girl directly onto your harddrive".