break·neck /ˈbrekˈnɛk/
  (a.)要使頸骨折斷似的,非常危險的
  Break·neck n.
  1. A fall that breaks the neck.
  2. A steep place endangering the neck.
  Break·neck a. Producing danger of a broken neck; as, breakneck speed.
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  breakneck
       adj : used of speed; "a breakneck pace"