Filler syllables. I hate them.
"Um..."
"Uh..."
"Ah..."
I hate hearing speakers use them, and even more, I hate hearing myself use them. It's a hard habit to break. However, this is emerging in a new way at my library. I recently noticed that several kids, including two I mentioned last year, are substituting it for a proper greeting.
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More appropriate ways to get someone's attention might be "excuse me," or, "pardon me," but kids these days [ahem, geezer's generalized grousing mode off] don't seem to be taught even the rudiments of etiquette. "Um" isn't how you open a conversation or start a sentence.
But at least it's not as obnoxious as The Girl Who Knocks. Then again, I haven't seen her in ages, and I occasionally wonder whether she's OK these days.