: very fast or dangerous breakneck speed.

What does break a neck mean?

: to cause one's neck bone to crack or break He fell off the roof and broke his neck. —sometimes used in speech to tell someone he or she could get hurt Stop jumping on the bed. You're going to break your neck.

How fast is break neck speed?

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.