Defintion of Hm- an word (or sound actually) used to verbalize such fellings as: Confusion, intrigue, anger, understanding, and several others.
ex. Jimmy says "can you pass the untrapulates?" you say "Hm?" meaning you don't understand what a untrapulate is (it is of course a made up word).
"Hm" is also used in as a synonym for many other words and phrases such as yes, indeed, please repeat your last statment, you make no sence, that is a good point but i don't fully agree, and in the case of repeating it several times consecutivly (hmhmhmhmhmmm) can be used as a synical or magnaminiacal laugh.
The word "hm" has many more uses than what i just listed, in fact i am sure there are more uses being implemented daily.
And that is why i like the word "hm", and use it daily.