After Jake crashes yet another sweet GTO and gets hauled in to the cop shop for questioning. The local cops and reporters, consequently, start using the phrase "Doyle for brains" whenever someone does something a little dense a la Jake Doyle.
Of course that Jake is at the heart of this embarrassing flare-up doesn't do anything for his reputation with his family members whose name is now being dragged through the mud. In an attempt to make amends with Mal, Rose, and Tinny Jake decides to find out who first started using the term and then to stomp out all subsequent uses.
Jake's blundering investigation just leads to more arrests and even more frequent use of the term. By the time Jake eventually gives up his quest to stomp out the term it is frequently used on Nightline and basically every other call-in show in the province every time someone wants to criticize the government of the day.