what does "flake on sb" mean?
I've heard a lot of time about this phrase recently,but I just can't figure out what it means

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It means to be unreliable. "He said he would come yesterday, but he flaked." Some people also say "flaked out".

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It means that person has dandruff flakes and that you shake their hair really hard so that all the dandruff flakes fall on to their shoulders so everyone can see them! :D