A person whose intelligence is impaired either permanently or temporarily. The latter condition is most often associated with unexpectedly encountering other road traffic whilst sitting in the driving seat of a motor vehicle. See also halfwitmoroncretinimbecileboy raceridiotmuppetteenager
"No of course I'm not upset that you pulled out of a side road right in front of me, thereby endangering myself and my entire family. Think nothing of it, Prune Brain!"