Urban Dictionary

A teacher's worst nightmare...

Or....A dictionary in which others compute their definitions of a word.

Definition 1
Teacher: Okay class, how did you guys find your definitions?

Most of the students: Urban Dictionary...
Teacher: Oh dear...where did I go wrong..

Definition 2
Gorge: Hey Bill, got any suggestions of where I can write my own definitions?
Bill: Urban Dictionary...Defiantly.