水洩不通 [－泄－－]

Jyutping seoi2 sit3 bat1 tung1
Pinyin shuǐ xiè bù tōng

Definitions (CC-CEDICT)
  1. lit. not one drop can trickle through (idiom); fig. impenetrable (crowd, traffic)
Definitions (Wiktionary)
  1. phrase, idiomatic
    densely packed; extremely crowded; impenetrable