出典:Wiktionary
Outgrowth of the earlier idiom, to hit the sack (“to lie down to sleep”), with possible influences from other senses of to sack (“to tackle, to pillage”), and to sock (“to hit, to slam”), providing an implication that sleep has been thrust upon a person.
「sacked out」は複合動詞「sack out」の過去形、または過去分詞です