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The opposite of overnight. If overnight is defined as the period from day through night to the next day, undernight is defined as moving from night through day into the next night.

Essentially means "daytime."

Synonymous with overday, if such a word existed.
"I'm staying at a friend's place overnight."

"Oh yeah? I'll be there undernight today, I might see you there before I leave."

"Word."
by Daem0n October 25, 2011
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