means The trait of acting without regard for the danger or consequences of what you're doing.

from From Old English 'reccan,' meaning 'to care for' or 'to heed' — the same root that gives us 'reck,' an old verb for taking notice. Add the negating '-less' and the noun-making '-ness,' and you get a state of literally not-caring: 'reck-less-ness,' the condition of heeding nothing. The verb 'reck' has faded to near-extinction (we mostly meet it now in 'reckless' and the poetic 'I reck not'), but it's a cousin of German 'geruhen,' to deign or condescend — both descending from a Germanic sense of paying mind to something. So recklessness is, at heart, an old word for the careless: a refusal to heed.