Which word describes an action taken rashly or headfirst?

The word that describes an action taken rashly or headfirst is "headlong." This term specifically conveys a sense of rushing into something without considering the consequences, emphasizing the quick and often impulsive nature of the action. It suggests an urgency and lack of caution, aligning perfectly with the idea of acting rashly or without thought.

In contrast, "heedless" implies a lack of awareness or attention to what one is doing, but it does not specifically communicate the physicality of rushing or diving in. "Haphazard" refers to something done in a careless or random manner, which lacks organization rather than implying a direct, headfirst approach. "Hazardous" describes something that is dangerous or risky, but it does not inherently carry the notion of acting rashly or quickly.