General synonyms:
* Carelessness
* Rashness
* Imprudence
* Foolhardiness
* Irresponsibility
* Negligence
* Thoughtlessness
* Headstrongness
* Daring (can be positive or negative)
* Audacity (can be positive or negative)
More specific synonyms:
* Bravado (recklessness stemming from a desire to impress)
* Temerity (recklessness bordering on audacity)
* Inconsideration (recklessness that disregards others)
* Wilful disregard (consciously ignoring the potential consequences)
* Impetuosity (recklessness driven by sudden impulse)
The best synonym will depend on the specific situation and the nuance you want to convey. For example, "reckless driving" might be better described as "negligent driving" or "careless driving," while "reckless disregard for safety" might be better described as "wilful disregard for safety."