A bunk is a bed that's built above another bed. Twin sisters might argue nightly over who gets the top bunk.
You might find a bunk bed in a child's bedroom or a train's sleeper car. Sometimes the rough bed in a rustic cabin is also called a bunk — and you could say, "I'll bunk in the cabin tonight." Informally, bunk is also nonsense: "How can you believe that bunk?" The bed meaning is short for bunker, while the "foolishness" definition is from bunkum, named for a politician's long, dull speech in 1820, in Buncombe, North Carolina.