嚴絲合縫 [严丝－缝]

Jyutping jim4 si1 hap6 fung4
Pinyin yán sī hé fèng

Definitions (CC-CEDICT)
  1. (idiom) to fit together snugly; to dovetail perfectly
Definitions (Wiktionary)
  1. phrase, idiomatic
    to fit together perfectly; to join tightly; to dovetail