In reply to:

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Shouldn't the right saying be that someone is "doing a job well"?

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Hell, the English language is often pretty crazy. I'm afraid it can mean either thing and the only way to work out which is the context in which it is said.

Just a little bit of it can bring you up or down.Like the supper it is cooking in your hometown.it is chicken, it is eggs, it is in between your legs.it is walking on the moon, leaving your cocoon.