The "Morning" for someone who is hungover and has been out partying. The afternoon for everyone else.

Apparently the phrase was spawned in awe of a rowdy kiwi guy named "Johnny" who went hard every night out and whose name is now legend and synonymous with kiwi drinking culture.
Son stumbles into the living room at 2:43pm
Dad: "Johnny morning son"
Son: "Errrrg I'm never drinking again"

and

Two mates wake up in a trashed hotel room with no memory
"Johnny Morning bro, what the fuck happened last night?"
"I don't know, lets go get a fucking bacon sandwich"
by Vagos June 20, 2013
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