An informal, friendly way to address someone, especially a man or boy; a casual form of address like 'buddy' or 'pal'.
From reduplication or onomatopoeia, possibly from 'brother' or simply an invented exclamation. The term became popular in American English in the early 20th century as a colloquial address.
Words like 'bub,' 'buddy,' and 'pal' are linguistic democracy—they flatten hierarchy by refusing titles and last names. When a tough guy calls you 'bub,' he's claiming solidarity with you, which is why it works in old noir films and why it sounds odd in formal contexts.
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