The word "hm" is generally used as an expression of hesitation, contemplation, or uncertainty. It often signifies that the speaker is thinking or considering their response before speaking further. It can also indicate a lack of understanding or curiosity about a situation.
The expression "hm" is believed to be an onomatopoeic word, imitating the sound made when a person is pondering or uncertain. It does not have a formal etymology as it is primarily a sound rather than a word in the traditional sense.