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1.
A statement or piece of news that everyone knows, or is out-dated. To describe such a statement as "yestersay" is like discarding it as "so yesterday".
Lisa: "Hey, did you hear that Jane Doe is dropping out of the Republican race?"

John: "Dude, that's yestersay!"
by nurdsmith January 05, 2012
 
2.
as in old news. when someone has just told you information that you already know.
Beni went all out telling me about the 13 year old father in the UK, but I knew about it already. This news is so yestersay!
by annie004 February 20, 2009
 
3.
Words spoken the day before, or prior.
"Just do it" is yestersay.
by PrairieDawg October 24, 2008