An unfashionable person, a person lacking style or character, a socially awkward adolescent, a "nerd".
These senses of dag derive from an earlier Australian sense of dag meaning a "character"., someone eccentric but entertainingly so.
Ultimately all these senses of dag are probably derived from the British dialect (especially in children's speech) sense of dag meaning a "feat of skill"..." a daring feat among boys", and the phrase.. to have a dag at...meaning "to have a shot at".
The Australian senses of dag may have also been influenced by the word wag (a habitual joker), and other Australian senses of dag referring to sheep (see rattle your dags below)
Dag referring to an unfashionable person etc; is recorded from the 1960s.
1983 Sydney Morning Herald 24 September...
Has it helped them feel more relaxed with the boys in their PD group?
"Well most of them are dags", Julie laughs, "but at least they're easier to talk to".
2011 Financial Review, Sydney 11 July...
Christian, while your budget may appear to be reasonable...your dress sense is nothing less than appalling. Never wear a striped suit, a striped shirt, and a striped tie together...just dreadful...You look like a real dag.
dag...rattle your dags
Hurry up, get a move on.
Dags are clumps of matted wool and dung which hang around a sheep's rear end.
When a daggy sheep runs, the dried dags knock together to make a rattling sound.
The word dag...originally daglock...was a British dialect word that was borrowed into mainstream Australian English in the 1870s.
The phrase is first recorded in the 1980s
1984 S. Thorne, Battler...
C'mon Mum, rattle yer dags...the old girls are hungry!
2010 Countryman, Perth 11 February...
Rattle yer dags, wool classers, times running out to re-register yourselves with the Australian Wool Exchange