/ ˈmʌkə(r); NAmE / noun
1.
( BrE , informal ) used when talking to or about a friend to refer to them :
It's my old mucker John!
2.
( NAmE , old-fashioned , informal ) a person who is rough and does not have good manners
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WORD ORIGIN
late 19th cent.: from muck + -er ; sense 1 probably from the phrase muck in . Sense 2 is probably from German Mucker sulky person.