Where "Dob In" Comes From
Refreshingly honest answer here: nobody's entirely sure. The most likely lead points to British dialect uses of "dob" meaning to throw or to put something down heavily — found in northern English and Cornish dialect. The trouble is timing: most Australian words with a clear British-dialect origin turn up in the second half of the 1800s, when that dialect influence on Australian English was strongest, but "dob in" and its variants are first recorded well after that window closed.
That gap doesn't rule the theory out, but it does mean the honest answer is "probably British dialect, origin unconfirmed" rather than a settled story. One very early recorded use appears in a horse-racing gossip column in Sport (Adelaide), 28 August 1914, which describes a "Dob-in-cottage" where "you can get a sackfull of news."
Source:
"He dobbed me in to the teacher."
How Aussies Actually Use It
Used from the playground right through to adulthood, whenever someone reports another person's wrongdoing — "he dobbed me in to the teacher" is a childhood classic, but the term works just as well for adults reporting a workplace rule-breaker.
"Don't be a dobber" is one of the earliest social rules many Australians learn, reflecting a broader cultural discomfort with informing on others.
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