Made something less sharp, bright, intense, or interesting; reduced the strength or clarity of something.
From Middle English 'dull' (probably from Old Norse), meaning slow or stupid; extended to mean 'not sharp' and later 'not bright' or 'not interesting' by the 15th century.
Your senses actually dull through adaptation—stare at something long enough and your brain literally stops sending signals about it, which is why background noise becomes invisible and why stores constantly change displays.
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