Why do guys get the slap after saying something (mostly whispered) to a girl?

I've seen this happen once or twice in real life and many times in movies.

I was always "shy and nice" when talking to girls in my younger years so I never figured out what would trigger a girl to slap a guy like this.

@girls: What did a guy say to u that made you slap em? Especially those whispers in the ear.

@guys: What did you say that got u slapped?

Why do guys get the slap after saying something (mostly whispered) to a girl?
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