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1.
A term used to describe the day of a brutal, painful hangover when you've finally recovered the following day.

The origin coming from the saying "I was a mess yesterday" when describing the day after a night of late partying, boozing, etc.
Luke: How was your weekend?
Joan: I went out all night on Saturday for my going away party and messterday I was just useless.
by Lrzewnicki October 07, 2013