Hmm — or sometimes hm orhmmm — ranks among the words we English speakers say the most, and yet we give it scant thought. It's never defined for us as children, is left out of all but the fattest dictionaries and seems barely a word at all. (It doesn't even have a vowel.) Nonetheless, we all manage to grasp hmm's vast range of connotations. Spoken with a rising intonation: "I didn't understand — say again?" Prolonged, with ample m's: "I'm thinking deeply about what you said." High-pitched: "Yes, maybe — good idea!" Uttered quickly and at a lower register: "I am dubious." What is this eccentric word, and where did it come from? Although it exists in many languages in a variety of forms, its roots are elusive. "I have no theory of its origin," said Anatoly Liberman, a linguist at the University of Minnesota and an expert on word origins. "Possibly it could have spread from French to English”¦ but you cannot trace it in any way as far as its distant history is concerned, because the word is so natural that it may have arisen at any time." In other words, who knows? Even the Neanderthals might have tossed it around. Hmm is technically categorized as an "interjection," along with the likes of um, huh, ouchand wow. It's also "sound symbolic," along with onomatopoeia words like plop, ping or oink— "except that it's symbolic of, really, nothing," Liberman told Life's Little Mysteries. "The first h-sound is simply a substitute for breath, and the second m-sound, since the mouth is closed, is symbolic of the fact that we're not quite sure what to say," he said. The pause filler indicates that we're temporarily speechless, but still engaged. The variety of tones we may take add subtle meaning to the interlude. Nicholas Christenfeld, a psychologist at the University of California, San Diego, and an expert on filled pauses, suspects hmm is popular primarily because it's such a neutral sound. That is, "it's easier to say than anything else," he said. Source
