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a room, closet, hollowed out wall, loose floorboard, or any other various type of empty space that can be used to hide illegal, secret, private, or otherwise undesirable items.
"yo, where are you keeping the beer?"

"in my room of requirement underneath the floorboard in my closet"
by PJ Seeds June 15, 2008
Duh! The restroom, what else?
Z: Where you off to?
A: The room of requirement
Z: For what?
A: Duh!
by Pseudonym, obviously August 14, 2008