The act of taking a solid 12 minutes alone and masturbating online.
E: Hey mate
E: you still here frend?
E: ..you been gone solid 12 minute, mate. You masturbating?
R: YES FREND I GOD DAMN TWELVING ALMOST DONE.
The act of faking being high. Pretending and acting like you're high when you really aren't.
Yo Josh is so stoned right now!
Nah dude he only took one hit he's twelving.
To stand a motorcycle or other vehicle straight up in the air, front over back.
In motorcycle stunting, pulling a wheelie so high that the bike rests on the it's tail section is called a "twelve o'clock". The act thereof is called "twelving".
To 'space out'. Specifically the special sort of mental sluggishness brought on by watching television or any similar mental haze.
1: "Which street do we turn onto?"
2: "Huh? Oh, sorry. I don't know. I must have been twelving."