When something plummets, it falls straight down very quickly, with nothing to slow it. A rock dropped from a cliff plummets to the ground below. A bird flying high in the sky suddenly plummets when it spots prey in the water. The word captures both speed and direction: straight down, fast.

People also use plummet to describe anything that drops suddenly and dramatically, not just physical objects. Stock prices can plummet when investors panic and sell everything at once. A student's test scores might plummet if they stop studying. A basketball team's chances of winning the championship could plummet after their star player gets injured.

Sailors and builders once used lead weights called plummets, attached to strings, to measure depth or check if walls were straight up and down. Because lead is so heavy, it drops fast and straight when you let it go. Over time, people started using the word to describe anything falling that same way: quickly and directly downward.