It is used to playfully express in agreement to something. It's an alternative phrase instead of saying "yes". The "yesh" means "yes", and the "mesh" is used to add playfulness as its supposed to be very silly and quite stupid like kindergarten humor.
Tyler: Are we still going to the party?
Jordan: Yesh mesh
Tyler: What the hell does that even mean?
Jordan: It means I am going
Tyler: Why the fuck couldn't you just say "yes"?
Jordan: Because it's more stupid this way
Tyler: Yes you are very stupid and are failing at life
Jordan: ok I'll see you at 5, bye, I go cry now
Jordan: Yesh mesh
Tyler: What the hell does that even mean?
Jordan: It means I am going
Tyler: Why the fuck couldn't you just say "yes"?
Jordan: Because it's more stupid this way
Tyler: Yes you are very stupid and are failing at life
Jordan: ok I'll see you at 5, bye, I go cry now
by NightWalker21 July 27, 2019