The word for which there are no words. Expresses extreme whateverness. To be "meh" is to be very "okay". Never to be used with other words though.
Besides this, meh can mean almost anything.
How was your day?
Bleh (Not good)
Eh (Not the best)
Meh (Okay I guess)
Will you be back soon?
Meh (Don't hold your breath)
Meh!! (same meaning as a regular meh, plus the extreme wanting of the person to leave)