A verbal expression of apathy, neutrality or indifference, equivalent to a shoulder shrug.
Originated from a Simpsons episode when Bart and Lisa used it to respond to a question from Homer.
Man: Hey, dude, it's an English final in the Champions League this year!
Man: But, aren't you excited? I thought you loved football.
Man: But Man U versus Chelsea!
Dude: God, man, what part of "meh" don't you understand?
On a drug like heroine, cocaine, ecstacy. Usually shortened to high. Usually people high up will exagerate emotions, so a small liking, can become love, and a small disapproval, can become total hate/disgust.