有頭威冇尾陣 [－头－－－阵]

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Definitions (CC-CANTO)
  1. starting strong at the beginning but not ending strong; to abort a mission halfway through
Definitions (粵典–words.hk)
  1. 語句
    虎頭​蛇尾​；​開​始​嗰​陣​係​威​係​勢​，​到​最尾​就​冇​晒​聲氣
    to start strongly but finish weakly; literally: impressive at the beginning but tailing off at end
Definitions (Kaifangcidian)
  1. 虎頭蛇尾 – 虎头蛇尾
  2. 有始無終 – 有始无终
  3. 有前勁沒後勁 – 有前劲没后劲