A small sound in the throat, written down. People make it when they are thinking, when they are not sure, when they need a pause, or when they do not quite agree but do not want a fight. The meaning lives in the tone, not in the letters. A soft hm can mean “let me think”. A short, flat hm can mean “I doubt that”. A longer hmm often means “I am still working it out”. In writing, people add m’s — hm, hmm, hmmm — to show a longer pause or a heavier doubt. It is not a word built from pieces, and it is not a name for a thing. It is the sound itself. In a text message, hmm can sound colder than you meant, because the other person cannot hear your kind voice; they only see the doubt. Though actually, English has a little family of these throat-sounds (uh, um, er), and hm is the one that most often carries “I’m not convinced” as well as “I’m thinking”.