/prɪˌsɪpɪˈteɪʃən/
[noun] Water that falls from the sky in any form — rain, snow, sleet, or hail — or, more generally, the act of doing something too quickly and without enough thought.
This word for rain and snow once meant 'throwing someone headfirst off a cliff' in ancient Rome.
The story of 'precipitation' begins on a rocky cliff above ancient Rome. The Latin word 'praeceps' meant 'headlong' or 'headfirst', built from two pieces: 'prae', meaning 'before' or 'in front', and 'caput', meaning 'head'. From 'praeceps' came the verb 'praecipitare', meaning to throw something — or someone — violently downward, head first. The Romans used this word in a very literal sense: condemned criminals were sometimes hurled from the Tarpeian Rock, a cliff on the Capitoline Hill, and this brutal act was described using the root that would one day give us our polite word for drizzle.
From 'praecipitare', Latin formed the noun 'praecipitatio', meaning a headlong fall or a rushing downward movement. This passed into Old French as 'précipitation', carrying the same sense of sudden, violent descent. English borrowed the word directly from French and from the original Latin around the late 15th century, and for its first century or so in English it was used almost entirely to describe reckless haste — the feeling of rushing into something without stopping to think, as if tumbling head over heels down a slope. A person could act 'with precipitation', meaning they were dangerously impulsive.
The word's leap into the world of weather came gradually and quite naturally. Scientists in the 17th and 18th centuries were working hard to describe the behaviour of liquids and vapours, and they noticed that water droplets in clouds do exactly what the Latin root suggests — they fall headlong downward under the force of gravity. By the mid-17th century, chemists were already using 'precipitate' to describe a solid that suddenly falls out of a liquid solution, settling to the bottom of a flask. This scientific usage spread the word's meaning, and meteorologists soon adopted 'precipitation' to describe any form of water falling from the atmosphere to the earth.
What is remarkable is that the chain from Latin to English preserves both strands of meaning almost perfectly. We still say that someone 'precipitates a crisis' — meaning they caused it to happen suddenly and carelessly — drawing on the original sense of reckless, headlong action. And we check the forecast for precipitation — rain, snow, or hail — drawing on the image of water tumbling downward from the sky. The same word covers both a panicked human decision and a winter snowstorm, because both share the essential idea of something plunging downward with speed and without hesitation.
The related word 'precipice' — meaning a steep cliff — travelled a parallel road from the same Latin root 'praeceps', entering English via French 'précipice' in the late 16th century, and it still wears its origin on its sleeve. 'Precipitous', meaning dangerously steep or rashly hasty, followed the same path. Even the word 'capital' shares a distant ancestor in 'caput' (head), showing how far and wide a single Latin root can spread across the English language over two thousand years.