An informal, sometimes playful or slightly insulting term for a man or friend, often used to address someone in a casual or tough way.
From Irish English 'bucaun,' possibly from Irish 'poc' (buck, male animal) or related to 'buck' (a young man). Popular in Irish-American slang from the 1800s onward.
This is pure Irish-American slang that made its way into English from working-class neighborhoods—your tough Irish uncle probably said 'Listen here, bucko!' The word captures a whole attitude and accent in three syllables.
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