If you are reckless, you don't think or care about the consequences of your words or actions, like a reckless driver who speeds while texting, knitting, and eating a sandwich.
The word reckless comes from the Old English word receleas, meaning "careless, thoughtless, heedless." If you have a reckless attitude, you aren't concerned about what happens to yourself or others who are affected by your actions. In this usage, reckless is the opposite of considerate. Reckless friends will invite people you don't know to your house because with your parents away, it's party time. Surprise!