Something that runs well or consistently; a horse or animal that runs fast, or an item that sells well.
From Middle English 'rennen' (to run), which comes from Old Norse renna. The -er suffix denotes something that performs the action of running, or in commercial use, something that 'runs' through sales successfully.
In horse racing slang, a 'renner' is a horse that's really fast, but modern retail workers also use it to mean a product that 'runs' off the shelves—same word, different contexts!
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