hmm

exclamation3

1. •
   a sound made by someone who is thinking about something

   • Hmm... that's an interesting idea.
   • Hmm... I just can't decide between the two.
2. •
   Indicates surprise or cluelessness; especially a response by a person who is questioned or addressed when not expecting it.

   • "You there! Where are you going?" "Hmm? Oh, I just need to get a drink."
   • "Could you pass me the paring knife?" "Hmm? What's a paring knife?"
3. •
   A demand for an answer to a question.

   • Just where were you until 2 a.m.? Hmm?

verb1

1. •
   To make a thoughtful humming noise.

noun1

1. •
   Initialism of hidden Markov model.