Mucker/ˈmʌkər/ Definition 1 of 4Pro Tip 1/3Slang Usage 'Mucker' is slang and very informal, used mostly in casual conversation among friends. I met a bunch of old muckers at the reunion. Pro Tip 2/3Affectionate Tone 'Mucker' implies a friendly, affectionate relationship when referring to someone. Don't worry, my mucker here will help us out. Pro Tip 3/3British Origin The term 'mucker' as a friend is primarily used in the UK and may not be understood elsewhere. Are you and Dave really close? Yeah, he's my mucker.