Breakneck describes a speed or pace so dangerously fast it seems like it might break your neck. When someone rides their bike at breakneck speed down a steep hill, they're going so fast that one mistake could cause a serious accident. When a company grows at a breakneck pace, it's expanding so rapidly that keeping up feels almost dangerous.

The word always suggests risk or danger along with the speed. A car traveling at a normal highway speed isn't moving at breakneck speed, but a race car careening around tight corners might be. If you're rushing through homework at a breakneck pace to finish before dinner, you're working so frantically fast that you might make careless mistakes.

Notice that breakneck is almost always used with words like speed or pace. You wouldn't describe a person as breakneck, but you could describe their actions or movements that way.