A mispronounciation of "bregin" originating in Guam in the 1950's by a retard named Froy Layvan (later made famous by being a subject in the experiments of Dr. Frink). Layvan tripped while saying 'bregin' and it came out 'gretchin' and thencefourth said 'gretchin' in it's place. In 1954 he was moved to the US for study where the saying caught on like wildfire.
