脣齒 [唇齿]

Jyutping seon4 ci2
Pinyin chún chǐ

Definitions (Wiktionary)
  1. noun, literally
    lips and teeth
  2. adj, figuratively
    intimately related; interdependent
Definitions (Kaifangcidian)
  1. （沒有對應漢語詞彙）