Other forms: blithest
The adjective blithe used to mean happy and carefree, but over time it has also come to describe someone who isn't paying attention the way they should.
If you have a blithe disregard for authority, you might just smile vaguely when a teacher is yelling at you and continue writing on the lockers with a Sharpie. If you're dancing to music while driving, and pass blithely through a red light, chances are you will be pulled over and given either a ticket or a lecture.