the highest gear in a vehicle that is used when you are driving fast
Idioms
in/into high gear
in a state of intense activity Her career is back in high gear. 她的事业重回正轨。
kick/move/swing into high gear
to become more intense The campaign season for this year’s elections doesn’t really kick into high gear until June. 直到六月，今年大选的竞选季节才真正开始。