Hmm (interjection)

1. Used to indicate that one is thinking about something or is uncertain about something.
2. Used to indicate that one is considering something or is not sure about something.

Origin:

Representing a sound made when one is thinking, hesitating, or uncertain.

Examples:

1. Hmmm, I'm not sure about that.
2. Let me think...hmm, no I don't know the answer.
3. Hmmm, that"s an interesting idea, I"ll have to think about it.
4. I"m trying to remember the name of that movie...hmm, it"s on the tip of my tongue.
5. I'm not sure what time it is...hmm, my phone seems to be dead.