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What does "uhm" mean? Does it mean "yes" (impolitely)? And is it a common word?
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I could mean 'yes' ('uh-huh'), it could mean 'no' ('uh-uh'), it could mean 'I'm listening' ('uh-huh'), it could mean 'what?' ('huh?'), it could mean 'I'm pausing because I don't know what to say, but I want to keep speaking' ('umm').

All are common, and it is difficult to know which utterance you are talking about without any context.
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It most often is what we cause an "articulated pause" -- that is, the speaker stops for a second or two but keeps on making a sound, usually to indicate that he is not through speaking. I have a linguist friend who has made a study of the most common articulated pauses in various languages and has come up with some interesting differences.
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Comments  
I see the word "uhm" here . But it seems that the author is not serious. By the way, the spelling is un-English, isn't it?
Do not rely upon the Urban Dictionary for any definitions. 'Um' is not a word; it is merely the transcription of a sound, so there is no spelling rule.
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Thank you, Mister Micawber Emotion: smile.
 Philip's reply was promoted to an answer.