The phrase has roots in both historical bedding practices and military slang:
: In the late 19th and early 20th centuries, mattresses were often just large cloth sacks stuffed with hay or straw. "Hitting the sack" was a literal description of laying down on these improvised beds. Hit the Sack
: While the term existed earlier, it surged in popularity during World War II. Soldiers frequently used "sack" as slang for their sleeping bags. The phrase has roots in both historical bedding