1
general::
Go to sleep, go to bed, as in We sacked out about midnight. This slangy idiom is a verbal use of the noun sack, slang for "bed" since about 1940; it alludes to a sleeping bag and appears in such similar phrases as in the sack, in bed, and sack time, bedtime.
American Heritage Idioms
2
general::
Phrase(s): sack out
to go to bed or go to sleep. • It’s time for me to sack out. • Let’s sack out early tonight.
McGrawhill's American Idioms And Phrasal Verbs