He's a hothead - he never thinks twice before acting.
Tips
usage
Colloquial and affectionate-to-mildly-scolding, not really an insult. The -鬼 suffix here is a productive familiar tag for 'a kind of person who...': compare 小气鬼 (stingy person), 胆小鬼 (coward), 醉鬼 (drunkard), 馋鬼 (glutton). 失 here is neutral tone.
memory
冒失 (màoshī) by itself = 'rash, abrupt'. Add 鬼 (literally 'ghost', here just 'guy/type') and you get 'the rash type' - a klutz.
