Causes a train to come off its tracks, or causes plans or thinking to go wrong.
From French 'dérail' (de- + rail), meaning to go off the rails. The prefix 'de-' means 'off' or 'away from,' combined with 'rail' from English railway terminology. The metaphorical sense of disrupting plans emerged in the 1800s.
The word 'derail' became a metaphor for disrupting thoughts and plans because train accidents were so catastrophic and sudden in the 1800s—when a train derailed, chaos erupted instantly, making it the perfect metaphor for when someone's argument or logic 'crashes.'
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