Most typically used as a noun to refer to oneself or others when totally drunk and can't remember a thing.
It can also be used as an adjective to describe onself or others when totally drunk.
"Man, I was totally Johnny Blackout last night. I don't remember a thing. What happened?!"

or...

"Damn, I was Johnny'd last night, fuck!"
by JohnnyBlackout October 9, 2008
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