From the Daily Telegraph (UK):
Meh, which can mean unimpressed, mediocre or boring, was chosen as the public's entry for the 30th anniversary edition of Collins English Dictionary which will be published next year.
People were asked to recommend a word to a panel of Collins language experts who chose meh because of the frequency of its use in today's English.
Meh was submitted by Erin Whyte from Nottingham who defined it as "an expression of utter boredom or an indication of how little you care for an idea".
Meh, which can function as adjective or interjection, gained popularity after the airing of an episode of The Simpsons ("Hungry Hungry Homer," 2001) that included this exchange:
Homer: (after watching Blockoland commercial) All right, kids ... who wants to go ... to ... Blockoland?
Bart and Lisa: Meh.
Homer: But the commercial gave me the impression that ...
Bart: We said meh.
Lisa: M-E-H. Meh.
However, the word first appeared much earlier, in a 1995 Simpsons episode. Read all about it in Benjamin Zimmer's definitive essay at Language Log, "Meh-Ness to Society." Also worth reading: Ben Yagoda's 2007 essay for Slate, "How the Internet Is Saving the Interjection," which discusses meh along with its interjectory relatives awwa, feh, and heh.