Definitions for sack out
go to bed, turn in, bed, crawl in, kip down, hit the hay, hit the sack, sack out, go to sleep, retire(verb)
prepare for sleep
"I usually turn in at midnight"; "He goes to bed at the crack of dawn"
To fall asleep, usually from implied exhaustion.
Origin: Outgrowth of the earlier idiom, to hit the sack, with possible influences from other senses of to sack (tackle, pillage), and to sock (hit, slam), providing an implication that sleep has been thrust upon a person.