Both Uses

impetuous
in
The Thundering Herd
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He put a hard curb on his impetuosity, and forced himself to wait and think carefully of every action before he undertook it.†
Chpt 12 *

impetuosity = impulsiveness (the trait of acting suddenly without much thought)
And when the order came to run down the canyon to prevent a possible massacre of Starwell and men, who had impetuously advanced too far, some were left behind.†
Chpt 16

impetuously = impulsively (acting suddenly without much thought)

Definitions:

(1) (impetuous as in: an impetuous decision)
impulsive (acting suddenly without much thought) -- often with an unfortunate consequence
(2) (meaning too rare to warrant focus)
In historic literature, impetuous often refers to any rapid, forceful, and/or violent movement.