The long flaps at the back of a formal jacket; also means benefiting from someone else's success without doing the work yourself.
From 'coat' (a garment) plus 'tails' (elongated ends). The literal meaning is the skirts of a 19th-century formal coat. The figurative 'riding on coattails' comes from the image of clinging to someone's clothing as they move forward.
The phrase 'riding on someone's coattails' is a perfect example of how English takes a literally visual image and turns it into a metaphor—you can almost see someone grabbing the back of a fancy jacket to get pulled along!
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