What's the meaning and origin of the Australian slang "fair dinkum"?

According to stereotype, spoken Australian English is thought to be highly colloquial, possibly more so than other spoken variants. Whether this idea is grounded in reality or not, a substantial number of publications aimed at giving an overview of Australian English have been published.

Many books about Australian vocabulary have been published, beginning with Karl Lentzner's Dictionary of the Slang-English of Australia and of Some Mixed Languages in 1892. Several similar books soon followed, including a relatively modest but authoritative work by E. E. Morris: Austral English: A Dictionary of Australasian Words, Phrases and Usages (1898).

After a long period of disinterest and/or antipathy, the first dictionaries of Australian English began to appear. In 1976, the Australian Pocket Oxford Dictionary was published, the first dictionary edited and published in Australia, by Graeme Johnston. In 1981, the more comprehensive Macquarie Dictionary of Australian English was published, after 10 years of research and planning. Updated editions have been published since and the Macquarie Dictionary is widely regarded as authoritative. A fuller Oxford Dictionary of Australian English has also been published.

Various publishers have also produced "phrase books" to assist visitors. These books reflect a highly exaggerated and often outdated style of Australian colloquialisms and they should partially be regarded as amusements rather than accurate usage guides.
A common feature of traditional Australian English was rhyming slang, based on Cockney rhyming slang and imported by migrants from London in the 19th century. Rhyming slang consists of taking a phrase, usually of two words, which rhymes with a commonly used word, then using the first word of the phrase the represent the word. In recent years this feature of Australian English has declined under the impact of mass popular culture.
Australians use a variety of colourful terms to refer to people. These terms may indicate such things as the person's ethnicity, the place where the person resides, the social status of the person, the person's behaviour, etc. Many of these words occur in other English dialects, especially New Zealand English, whilst others are unique to Australian English.
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