To go to bed or go to sleep. It's an informal way of saying you're ready to retire for the night.
This American phrase dates to the early 1900s and comes from the time when mattresses were often just sacks filled with straw, hay, or other materials. 'Hitting the sack' literally meant getting into bed by plopping down on these simple, sack-like mattresses.
The phrase captures the ungraceful reality of exhaustion - when you're truly tired, you don't gracefully 'retire' to bed, you 'hit' it with the force of someone who can barely stand anymore. The verb 'hit' suggests both the physical impact and the sudden transition from wakefulness to sleep.
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