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This page is about the slang term sack (2).
Meaning: a bed
I'm gonna hit the sack, so I'll see you in the morning.
Andy loves Sundays because he can stay in the sack all day if he wants to.
Note: Only used with the definite article "the", and only in common expressions such as "hit the sack". It's not used when talking about a bed as a piece of furniture, so we would not say "I'm buying a new sack today". We'd have to say "I'm buying a new bed today."