If you get lazy because of your overconfidence, you'll probably be prone to negligence. You start cutting corners everywhere because you think you're infallible, and before you know it you're neglecting all the little things you were supposed to take care of.

怠慢 refers to the kind of negligence where someone fails to fulfill a duty. It suggests they could have done better but didn't because of laziness, carelessness, lack of effort, or not taking their responsibilities seriously.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.