不潔
ふけつ · fuketsu · JLPT N2 · na-adj
unclean
🦠 不 (not) + 潔 (clean, pure) = not clean = unclean; 不 negates the 潔 in 清潔 (cleanliness)
潔 (けつ, ketsu) means clean or pure, as in 清潔 (せいけつ, seiketsu, cleanliness). Putting 不 in front negates it, giving 不潔 (ふけつ, fuketsu), unclean or dirty.
不 — single pictograph; bird flying upward
Originally a pictograph in oracle bone script, traditionally interpreted as a bird flying up toward the sky and unable to come down, suggesting negation or refusal. Over time it became the standard negation word in Chinese, used before verbs and adjectives to mean 'not'.