What it means
To hit the sack is to go to bed, the very same idea as hitting the hay, since the sack was the old straw-stuffed mattress or bedroll you flopped onto. It is casual and a bit weary, what you say when you are dog-tired and ready to crash out for the night. Right, I am going to hit the sack, see you in the morning.
Usage examples
"I'm shattered, I'm going to hit the sack before midnight for once."
"The kids were so tired after the party they hit the sack without a fuss."
"Early start tomorrow, so I am going to hit the sack now."
Where it comes from
Straight from the days when a bed was a sack stuffed with straw or hay, the very same image as "hit the hay". To hit the sack is to flop down onto it and turn in for the night, usually once you are well past ready for sleep.
Other ways to say it
Your vote counts
Is this real street talk or have we lost the plot? Cast your vote.