The long back flap of a formal coat (like a tuxedo), or to metaphorically ride on someone else's success without earning it yourself.
English compound: 'coat' + 'tail' (from Old Norse 'tagl'). The metaphorical meaning 'to ride on another's success' emerged in 19th-century American politics, as in 'riding someone's coattails.'
The phrase 'ride someone's coattails' comes from the image of a child or subordinate literally holding onto the long tails of a powerful person's formal coat—it's a perfectly visual metaphor for following someone's success!
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