Device used to notify others that they are in the presence of a dickhead. The point may be reinforced by the wearer displaying said item at alternative angles, particularly back-to-front especially when facing the sun.
That wanker in the baseball cap.
Device which, when fitted to the head of anyone driving a car, immediately removes any concern they may have previously exhibited for their own, their passengers, and, well, anyone elses safety.
Especially effective when driving 'modded cars'. See kev.
Tracy thinks that Trevor's baseball cap makes him a much better driver.