"Of course, he might have crawled into the airing cupboard and died... but I mustn't get my hopes up..."
-- Sirius Black, speaking of Kreacher (OP23)
An airing cupboard is a cupboard for airing linen and clothing (NSOED). Similar to U.S. “linen closet.”
Sirius supposes that Kreacher might have curled up and died in the airing cupboard at 12 Grimmauld Place (OP23).
An airing cupboard is a type of linen closet that typically contains a water heater and has slatted shelves to allow the heat from the boiler to circulate throughout the cupboard, keeping the stored linen free from damp.