A dirtbag is someone you think is very unpleasant and has no morals — a person who treats others badly and does not care. It is American slang, rough, and it is an insult. You might hear it in films and online. You would not put it in a school essay, and you would not say it to someone’s face unless you meant to hurt. The picture in the word is unkind: the person is being called a bag of dirt. “Bag” here is the same rude pattern as in scumbag. Though actually, climbers and some outdoor people also use dirtbag in a half-proud way, for someone who lives cheap and messy so they can spend their life on the rock or on the road. That second use is in-group slang. The first use — a bad, dirty person — is the one most learners will hear as an insult, and it is the one to be careful with.