German for "driving black"
In other words, riding on the public transportation, or trains without a ticket.
Ticket checkers with randomly come to check your ticket on the train, and if you do not have a ticket, or have a ticket that is not date stamped, you will be issued a big fine.
Also, note: That the student tickets for the U-bahn in Germany are not for students under 18, they are specifically for students from the grades kindergarten through sixth grade... (Strange but I got in trouble by accidently buying a student ticket when it wasn't for any student...)
"Bist du schwarzfahren?" "Nein, habe ich meine Fahrscheine vergessen, aber kann ich für einmal bezahlen?" (and sometimes if the man/woman is nice they will let you pay the price for the ticket instead of giving you a fine.)