For years, teachers have warned against using the word ain't, apparently with some success. Emily Hummell from Boston sent us a poem that may have contributed: "Don't say ain't / your mother will faint / your father will fall in a bucket of paint/ your sister will cry / your brother will sigh / the cat and dog will say goodbye." This is part of a complete episode.
My memory is a bit hazy, but I believe I ran across something related in Dorothy Sayers, in one of her Lord Peter Wimsey novels, spelled imphm, or possibly imphim, to indicate the casual nasal affirmative sound that results from saying "uh-huh" (meaning yes) with the mouth closed.
Or it could be the kind of thoughtful descending hum that might indicate "Oh – yes, I think you're right" or "Okay, I get it now."
In either case, it's an attempt to spell a sound that conveys meaning without actually being a word, if such a thing is possible.