水垢

shuǐgòu
limescale

CEDICT

Entry Composition

1 水 shuǐ
  1. water
  2. (after a name) ... River
  3. to swim (used mostly in 会水 huìshuǐ and 水性 shuǐxìng)
  4. (coll.) lacking in substance; shoddy
  5. (bound form) additional cost; extra income
  6. classifier for washings of a garment
2 垢 gòu
  1. dirt
  2. disgrace