“Meh” is used to show one’s indifference, boredom, and most commonly, a state of being utterly unimpressed with something. The reason it rankles is because it implies a relationship to the world in which the writer is a passive consumer, and everything else exists to entertain and interest him. Basically, he expects everything he consumes online to align with his own personal standards, interests, and tastes. If these inflated expectations are not met, he is disappointed. “I, the emperor of one, am not well pleased! Send in another jester!”
If something does actively disappoint me, I am not inclined to comment on it. And that’s my beef with “meh.” If something bored you, and you don’t wish to engage with it, why bother saying so? “Meh” expresses indifference, but an indifference not thorough enough to resist making itself known. As John Hodgman put it, meh “universally seems to signal: I am just interested enough to make one last joyless, nitpicky swipe and then disappear.”
Hodgman also touches on another unfortunate aspect of meh: “It’s part of the toxic Internet art of constant callous one upsmanship.” Someone will convey some kind of passion, enthusiasm, or interest in something, and someone else will come along and pour a cold bucket of water on that fire with their droll “meh.” A “meh” says “I’m too cool/intelligent to get worked up about something as dumb as this. Too bad you haven’t attained the same level of sophistication as me.”
In short, “meh” aims to show one’s indifference to something based solely on its lack of alignment with one’s personal taste. It’s what you reach for when you lack the discipline to remain silent, or are too lazy to make an argument, and no thumbs up, thumbs down button is within reach.
Excerpt from 3 Ways You Should Never Start an Online Comment