"meh"
tolerable written, strangle worthy said out loud like it's a word
I use that one quite a bit, both as a written word and as a spoken word. "Meh" isn't a noun, a verb, pronoun, adverb, adjective, preposition, conjunction, determiner, or an exclamation.
I think it drives some people batshit because it doesn't neatly fall into the easily identifiable nomenclature of word hierarchy. I think the category that the word would best fall into would be as an onomatopoeia. Being a word that that serves only the purpose of mimicking a sound.
That sound; the sound we often make when we raise our shoulders and shrug in a state of indifference to something someone says, or to an event that is happening.
"Meh" doesn't mean anything, there is no proper definition, and it's not a human universal expression in it's own right. Onomatopoeia's are universal to human speech in that each language has their own variants of them, so if you find it hard to find the idea behind "meh" in English, it'd have an arguably more vapid response from someone who struggled with English.