Top definition
When you can't remember what day something happened, because you've been awake for too many consecutive days.
Person 1: Remember that party yesterday... wait, or was that this morning?

Person 2: I don't even remember anymore, it was yestermorrow.
by TheReminder June 12, 2011
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2
Between yesterday and tomorrow; in other words, today.
We're having a party tonight, because my birthday is yestermorrow.
by OhcaptainMycaptain November 24, 2009
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3
When some idiot mixes yesterday and tomorrow together!
his birthday was Yestermorrow.
by KXLB3 August 21, 2006
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