As Cormac McKeown, head of content at Collins Dictionaries, says:
“This is a new interjection from the US that seems to have inveigled its way into common speech over here. It was actually spelt out in The Simpsons, when Homer is trying to prise the kids away from the TV with a suggestion for a day trip. They both just reply ‘meh’ and keep watching TV; he asks again and Lisa says, ‘We said MEH! – MEH, meh!’'
More here -- if you care. Meh!