Source : Webster's Revised Unabridged Dictionary (1913)

Breakneck \Break"neck`\, n.
   1. A fall that breaks the neck.

   2. A steep place endangering the neck.

Breakneck \Break"neck`\, a.
   Producing danger of a broken neck; as, breakneck speed.

Source : WordNet®

breakneck
     adj : used of speed; "a breakneck pace"