what does "a mean martini"really mean?

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Hi friends,I was reading an article just now,and its title is "I make a mean martini",I got confused after I have finished reading it because the whole article has nothing to do with the martini..or rather it doesn't even mention the martini...I'couldn't figure it out what does people really mean by saying"I make a mean martini or I fix a mean martini"..may you help me,please?Thanks in advance and wish you a very nice day.