what does "a mean martini" really mean?

[Infin1ty]

Hi friends,after finished reading an article entitled "I make a mean martini",I got confused because the whole article has nothing to do with 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.
:D
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[Jamie (K)]

"I make a mean martini," or, "I fix a mean martini," means, "I can make a very good martini."

[Infin1ty]

Could this sentence is used for describing that someone has done someting good or something else? Thanks for your help :D
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Please feel free to correct me if you find any mistakes in my posts.

[Torsten]

Hi,

I don't think it's an idiom.

TOEIC listening, photographs: The bull fight