"don't speak ill of dead"
I already know from dictionary that it is used when you are say bad things about someone. But I kind of thinking it is more specifically used when you are saying bad thing about a person who is dead and is not alive to defend herself/himself. Am I right here?

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Correct - we say it when we are talking about a dead person

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I think it's used specifically about dead people, not in general. Maybe it can apply to concepts or be used in an extended way to people who are dead figuratively (i.e. ex celebrities) as a joke or pun, but it's about dead people.

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The dead can't answer criticism so perhaps that is the meaning of the phrase, that and it's bad to speak ill behind someone's back.

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It's a very old saying--it is actually a translation of the ancient Latin "De mortuis nil nisi bonum," and I just learned from Wikipedia that THAT is a translation from the ancient.