General synonyms:
* Careless
* Rash
* Imprudent
* Inconsiderate
* Thoughtless
* Headstrong
* Impulsive
* Foolhardy
* Daring
* Adventurous (in a negative sense)
More specific synonyms:
* For actions: Irresponsible, heedless, wild, uncontrolled, haphazard
* For people: Brash, audacious, daredevil, devil-may-care
The best synonym will depend on the specific situation and the intended meaning.