Etymology

dumb as a doornail

Today it means: Irrefutably or unquestionably dumb. Full definition →

Middle English dom as a dore-nayle
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The story

  1. Middle English · inherited fromdom as a dore-nayle
  2. English todaydumb as a doornail

From Middle English dom as a dore-nayle.

Etymology from Wiktionary, CC BY-SA 4.0.