- The definition of breakneck is something that is moving really, dangerously fast.
Driving at 100 miles per hour is an example of driving at breakneck speed.
One car passes another at breakneck speed.
- Dangerously fast: a breakneck pace.
- Likely to cause an accident: a breakneck curve.
(comparative more breakneck, superlative most breakneck)
- A fall that breaks the neck.
- A dangerous steep place from which one could fall.