Mocking or making fun of someone or something in a rude, dismissive way, or eating quickly and greedily.
The mocking meaning comes from Scandinavian languages (Old Norse 'skaup'). The eating meaning is separate, possibly from British slang, with uncertain origin but in use since the 1800s.
This word has two completely unrelated meanings that confuse people: you can 'scoff at' someone's ideas (mock them) or 'scoff down' a pizza (eat it quickly). These don't share a common root, so it's just a coincidence that English uses the same word for contemptuous mocking and greedy eating!
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