It's used when someone simply doesn't care, doesn't know, or if they're feeling depressed/dejected. Accompanied by a shrug of the shoulders. Very similar to bleh and blah.
"Meh... the world sucks."
"How was your day?" "Meh..."
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Used when someone doesn't care, often when they're in a down sort of mood.
"I flunked my math final today... oh well..."
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