Cocktail served in a copper mug, familiarly (4)

I believe the answer is:

mule

(Other definitions for mule that I've seen before include "Illegal drugs courier" , "Offspring of a donkey and a mare" , "Sterile offspring of donkey and horse" , "Equine hybrid" , "Donkey and horse cross" .)

I've seen this clue in The New York Times.

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