1. (verb) To charge recklessly towards almost certain defeat. Very rarely does one "mushmom" and win.
2. (noun) Monster in the game Maple Story.
Comes from the online game Maple Story. During a week in August of 2005, some events were unleashed where monsters attacked the town within the game. During one particular part of the event, a bunch of n00bs were wandering around a town when a monster called Mushmom attack. (Sidenote: Mushmom is one of the stronger monsters in the game that only very elite people can kill.) The n00bs, believing they had skillz, decided to bond together for one moment and attack Mushmom all at once. Someone shouted "CHARGE HER!" and all the n00bs rushed forward...and died moments later, not even harming the giant mushroom in the process.
"Then we mushmommed him-and he completely destroyed us!"
When worse came to worse, the party had to mushmom the beast.