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1.
Adjective phrase, meaning not being in a state of one's normal mind. This could mean really tired, confused, insane, etc.
Joe: John just woke up, and man, he's out of it!
by USS Vagrant January 05, 2006
 
2.
(UK Definition)

A colloquial, informal term for "outrageously drunk", usually used in the past tense.
Tom was absolutely out of it last night!
by 84th December 08, 2011