Dag can mean several things:

1. In mathematics, Dag or DAG is an abbreviation for a directed acyclic graph.
2. In Norse mythology, Dag is an ancestor of Ottar.
3. An Australian term, meaning many things. According to the Macquarie dictionary, dag originally referred to "the wool on a sheep's rear quarters, often dirty with mud and excreta". Sheep poop in other words. "Rattle your dags" has been used to urge kids to hurry up. Eventually dag came to describe a person. The qualities being evoked range from conservative, unsophisticated and slovenly, to eccentric, goofy, amusing and loveable. To attempt the translation: dag might encompass the equivalent of anything from geek to slob to nut. Calling someone a dag might call forth any combination of these qualities. Along with the Australian proclivity to show affection via insult, this causes some difficulty in explaining the intended meaning to others (Americans). Much is taken from the context and the tone of voice in which it is spoken.