-  The first English speakers to refer in writing to the kangaroo were Captain Cook and the botanist Joseph Banks, who both mentioned it in 1770 in the journals they kept of their visit to Australia (Banks, for instance, referred to killing ‘kangaru’). This was their interpretation of ganjurru, the name for a large black or grey type of kangaroo in the Guugu Yimidhirr language of New South Wales. English quickly generalized the term to any sort of kangaroo, although it caused some confusion among speakers of other Australian Aboriginal languages, who were not familiar with it: speakers of the Baagandji language, for instance, used it to refer to the horse (which had just been introduced into Australia). There is no truth whatsoever in the story that the Aboriginal word was a reply to the English question ‘What’s that?’, and meant ‘I don’t understand’. The element -roo was used in the 19th century to produce jackeroo, which denoted ‘a new immigrant in Australia’, and is first recorded as an independent abbreviation of kangaroo in the first decade of the 20th century. The term kangaroo court ‘unofficial court’, which dates from the 1850s, is an allusion to the court’s irregular proceedings, which supposedly resemble the jumps of a kangaroo.
The Hutchinson dictionary of word origins. 2013.