Synset

characterized by undue haste and lack of thought or deliberation
<adj.all>   (a)   freq. cnt.: 2 freq. cnt.: 2

brainish

hotheaded

impetuous

impulsive

madcap

tearaway

sample sentences

"a hotheaded decision"
"liable to such impulsive acts as hugging strangers"
"an impetuous display of spending and gambling"
"madcap escapades"
(`brainish' is archaic)