1. Mother was late for a doctor’s appointment and the children were slow getting into the car. “ Step on it!” she snapped. “We’re late. ”
2. As the ambulance left for the hospital with my sick father in the back, I asked the ambulance driver to step on it. I wanted him to waste no time getting to the hospital.

Synonyms: shake a leg; get a move on. Antonyms: hold (one’s) horses; keep (one’s) shirt on. The expression is often used when the speaker is annoyed or short-tempered, as in opt.1, and probably stems from the idea of pressing down hard on the gas pedal to make a car go faster.