A dag is someone who looks stupid and uncool--a dork.
an Australian and New Zeland word meaning 'in bad condition'
It derives from the word 'dag' which means a piece of shit caught in the sheep's wool near its rear end--which can sometimes get fly-blown.
(Flies can lay their eggs in the dags and maggots will hatch. That's why sheep have to be crutched--the wool round the rear end is shorn to prevent this from happening.)