What an earth does 'It's no mean feat', actually mean, and where does it come from?

Question

Answer

in this phrase "mean" can indicate small or average.
feat is a task or something that needs to be accomplished.
So using those definitions, 'its no small task.'  However, that statement is meant to be a euphemism or understatement so the implied meaning is: 
"It was a great accomplishment" or "It was a great achievement."

Also, I think for your question, you meant to say
"what ON earth does .... "

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