1
general::
Go to sleep, as in I racked out about midnight. This slangy expression, as well as the related rack time, for sleeping or snooze time, use rack to refer to being laid out on a framework, here a bed.
American Heritage Idioms
2
general::
Phrase(s): rack out
Sl. to Go to bed and to sleep. • I’m really tired. I’ve got to go rack out for a while. • I racked out until nearly noon.
McGrawhill's American Idioms And Phrasal Verbs