Meh is an interjection, often used as an expression of indifference or boredom. It can also be used as an adjective (meaning mediocre, boring, or apathetic. Some have speculated that meh's origin is Yiddish because of its similarity to the interjection 'feh.'
The problem with tracing meh over time is that it's terribly underrepresented in the linguistic and lexicographical literature. A variant of meh, namely mnyeh, famously appears in the works of the late academic and humorist Leo Rosten. But Rosten's work is often disparaged by experts in Yiddishology, and, in any case, there's no evidence that mnyeh is even a Yiddish word.