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1. Extremely, often excessively, fast. If he can maintain that breakneck pace, I think he's going to win the race! You don't have to drive at such a breakneck speed—it's OK if we're a few minutes late.
2. Very extreme or dangerous. I'm not surprised that the train derailed at that spot where the tracks make a breakneck turn.
break one's back
Also, break one's neck. Make a great effort, work very hard. For example, I've been breaking my back over this problem for the past week, or Don't break your neck to get there; we'll wait for you. Both versions of this expression, polite equivalents of break one's ass, transfer the literal fracture of one's back or neck to figurative exertion. However, break one's neck has the secondary connotation of proceeding with reckless speed, a sense also conveyed by the term breakneck pace. Originally this idiom alluded to literally breaking one's neck by rushing heedlessly along, but it has been used figuratively for the past 300 years. Also see break the back of.