Thick, stubborn dirt or soot that builds up on surfaces over time.
From Old Norse 'grim' meaning 'mask' or 'phantom,' possibly because grime makes things look dark and unrecognizable. The word evolved to mean any thick, stuck-on dirt by the 1600s.
London's famous 'Great Smog' of 1952 actually created visible layers of grime on every surface—it was so thick that people couldn't see across the street, and this disaster led to the world's first air pollution laws!
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