An Australian slang term.
A dag is technically the matted wool on a sheeps tail, but in typical useage throughout Australia, it refers to people who don't have a neat, tidy or cultured appearance. It can also refer to a person who tends to be quite informal.
It is not necessarily a derogatory term in modern useage.
It can also refer to a Person who is a bit zany in antics in a loveable way
I'm a bit of a dag today, I'm wearing my trackie-daks