impetuosity

impulsiivisuus The quality of making rash or arbitrary decisions, especially in an impulsive or forceful manner.
(RQ:Fielding Tom Jones)

She had however another motive, beside her obedience, to accompany the old gentleman in the chase; for by her presence she hoped in some measure to restrain his impetuosity, and to prevent him from so frequently exposing his neck to the utmost hazard.