breakneck

ˈbreɪknɛk

adjective

dangerously or extremely fast.

he drove at breakneck speed

extremely fast,high-speed,lightning,whirlwind,rapid,speedy,dangerously fast,reckless,dangerous,excessive,precipitate,headlong,at full speed,at full tilt,at full pelt,flat out,ventre à terre,hell for leather,at a lick,like the wind,like a bat out of hell,like a bomb,like greased lightning,like the clappers,at a rate of knots,like billy-o