Spreadbury is the vein on your forehead that throbs and sticks out when a person enters rage mode.
Spreadbury evolved from a teacher (Mr. Spreadbury) who became so angry, he screamed out insults such as 'Blithering Idiot' at a ginger kid for pretending to play the drums during a lesson.
Mr. Spreadbury: "Okay class, be quiet..."
(McGriz bangs pencils on table)
Mr. Spreadbury: "YOU BLITHERIN' IDIOT!!! You could have stopped when I looked at you, but no, you just kept on playing!!!"