Dirt is loose earth or soil, especially when it makes things unclean. It can also mean any kind of filth or unwanted substance on a surface.
From Old Norse 'drit' meaning 'excrement', which over time softened in meaning to general filth or unclean material. The sense expanded further to mean simple soil or earth in many contexts.
Originally, 'dirt' was much nastier than just soil—it meant something closer to 'poop'. Language politely cleaned the word up over time, so now we can talk about 'dirt' on our shoes without blushing.
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