A popular caucasian men's haircut in the early 90's that got its name from the overall shape of the doo. This haircut did not survive the test of time and is frowned upon today. If you see anyone currently rocking this haircut you might want to give them the memo. There was a brief hybrid version that surfaced about a year later, in which the underbelly of the shroom was shaved. It was popular amongst the alternative and punk kids.
Hey fun-guy, you might want to shave "mushroom haircut".