The intricate drawings, reminders, and cheats ballpoint- inked onto one's hand. Often found on the hands of stressed people, bored people, or a combination of the two: students.
1: Hey, what's that on your hand?
2: Penna
1: What?
2: Penna. Like henna, but with a pen.
1: Cool!
OR
1: Dude, I didn't know you had a tattoo?!
2: I don't.
1: Then what's that?
2: Penna!