precipitate, precipitous

1. The two words overlap in meaning and were used interchangeably from the 17th century to the 19th century. Precipitous has a physical meaning 'sheer like a precipice':

There was a precipitous wooden stair to the ground floor—A. Craig, 1990.

In its abstract sense it is concerned with the over-rapid progress of an action and retains the notion of steep descent, and is therefore often found in the company of words such as decline

(A number of factors might be responsible for such a precipitous decline—A. Wilentz, 1989)

, whereas precipitate is concerned rather with the inception of an action and means rather 'hasty, rash, inconsiderate' or 'headlong, violently hurried':

His precipitate action was clearly calculated to make life harder rather than easier for the PLO as he abandoned responsibility for civil servants in the West Bank.—D. McDowell, 1990

One can't help wondering whether rumours of his precipitate departure might not be wishful thinking—Sunday Herald, 2001.

It is in this second set of meanings that the two words come closest, since any action that is precipitate in its inception is likely to be precipitous in its performance or consequences.

2. Of the corresponding adverbs, precipitously encroaches on precipitately, especially in American English:

I left precipitously because I didn't want to work there any longer—A. Cross, American English 1986 (precipitately is wanted)

Angus had precipitately fled on learning that the king was loose and in vengeful mood—J. Burke, 1990.

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