With a common origin in Latin terms meaning "to cast down," these words have taken on dissimilar meanings. As an adjective, precipitate means "headlong," "moving rapidly and hastily," "rash": "Take your time; don't make a precipitate decision." "Braking too hard will cause a precipitate stop." Precipitous means "steep," "abrupt," "perpendicular," "sheer" and is usually applied to mountains, cliffs, and all steep places: "Some of the streets of San Francisco seem almost precipitous." "That precipitous cliff is nearly 1,000 feet high."
Dictionary of problem words and expressions. Harry Shaw. 1975.