Michael Ryan Pritchard (born May 4, 1972), better known by his stage name Mike Dirnt, is an American musician, singer, and songwriter. He is best known as the co-founder, bassist, backing and occasional lead vocalist and former guitarist of Green Day. He has also played in several other bands, including the Frustrators. His stage name was originally a nickname that his friends from grade school gave him, as he constantly played "air bass/guitar" and made a "dirnt, dirnt, dirnt" noise while pretending to pluck the strings.

Further Reading: Wikipedia