Smelling stale, musty, or moldy; old-fashioned or conservative in thinking; stuffy or lacking in freshness.
From Middle English fust meaning a strong musty smell or stale tobacco, possibly from Old Norse or Germanic roots meaning to ferment or decay. The word shifted from describing actual smells to describing attitudes and ideas.
Fusty perfectly captures how English speakers link bad smells to bad thinking—if your ideas are 'fusty,' you literally smell old and rotten, which is why it became an insult for anyone hopelessly out of date.
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