The Collaborative International Dictionary of English v.0.48:

Breakneck \Break"neck`\, a.
   Producing danger of a broken neck; as, breakneck speed.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Breakneck \Break"neck`\, n.
   1. A fall that breaks the neck.
      [1913 Webster]

   2. A steep place endangering the neck.
      [1913 Webster]

WordNet (r) 3.0 (2006):

breakneck
    adj 1: moving at very high speed; "a breakneck pace"

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "breakneck":
   abrupt, accident-prone, agile, bluff, bold, breathless, careless,
   dangerous, daredevil, dashing, desperate, devil-may-care,
   double-quick, eagle-winged, excessive, expeditious, express, fast,
   fleet, flying, furious, galloping, hair-trigger, harum-scarum,
   hasty, headlong, hell for leather, hotheaded, hurried, hustling,
   impetuous, impulsive, light of heel, light-footed, lively, mad,
   mercurial, nimble, nimble-footed, overeager, overenthusiastic,
   overzealous, panting, plunging, precipitant, precipitate,
   precipitous, prompt, quick, quick as lightning, quick as thought,
   rapid, rash, reckless, running, sharp, sheer, slap-bang, slapdash,
   snappy, spanking, speedy, steep, sudden, swift, vertical, wanton,
   wild, winged