From Old Norse 'drit' (excrement), broadened in English to mean any filth or loose earth — a dramatic euphemistic shift.
A substance such as mud or dust that makes a surface unclean; loose earth or soil.
From Old Norse 'drit' meaning 'excrement,' from Proto-Germanic *dritą. The word originally meant specifically excrement — the generalization to any unclean substance happened in English. Key roots: *dritą (Proto-Germanic: "excrement").
'Dirt' meant excrement until about the 15th century. When you call something 'dirty,' you're using a very old euphemism — the original meaning was much worse.