Captain Cook landed in Far North Queensland in 1770, pointed at one of those weird hopping things, and asked the locals what it was called. The Guugu Yimithirr men shrugged and said 'kangaroo,' which in their language meant 'I don't understand the question.' Cook wrote it down, brought it back to England, and an entire species got stuck with a misunderstanding for a name. Every Australian primary school teacher has told this story. It's the sort of thing colonial history was made of.
It is the actual word. 'Gangurru' in Guugu Yimithirr is the term for a particular species of grey kangaroo, the eastern grey, and Sir Joseph Banks's notebooks from the 1770 Endeavour landing at the Endeavour River are clear that the word was given as the name of the animal in front of them. The 'I don't understand' story was already circulating by the 1820s and was repeated as fact by writers who didn't speak any Australian language and assumed the locals couldn't have either. The story was put to bed when linguist John Haviland did fieldwork with Guugu Yimithirr speakers in the 1970s and confirmed gangurru as the standing word for that species. Other Aboriginal languages have entirely different words. Cook didn't get bamboozled. He just asked the right people the right question and wrote down the right answer.