Skipping part or all of a day at school.
Often to go smoke or do drugs.
Usually punished quite heavily by schools e.g 2 after school detentions for every period wagged.
"How'd ya get so many dets?"
"For wagging a whole day!"
In Australia it is used usually to describe a Greek, Italian or Leb, as these are the most common ethnics.
"Oh did you see Carlos out there with his Monaro?"
"Oh yeah that FOB!"