breakneck

break·neck (brāk'nĕk')  
adj.

1. Dangerously fast: a breakneck pace.
2. Likely to cause an accident: a breakneck curve.

breakneck

adjective

Characterized by great celerity: expeditious, fast, fleet, quick, rapid, speedy, swift. Informalhell-for-leather. Idioms: quick as abunnywink. See fast/slow/velocity.