The word
kangaroo comes from a northern
Queensland language called Guugu-Yimidhirr, and does indeed mean kangaroo. It does
not mean "I don't know", though this is a widespread legend.
As explained under Guugu-Yimidhirr, this was the language encountered by Cook and Banks when they first landed, and they took the local name as the generic Aboriginal name for the animal. In fact, it means specifically the black kangaroo; and also the Guugu-Yimidhirr kangurru could be written down variously in English, giving rise to the modern English pronunciation (with a G as in finger, whereas the original was as in singer).
So when years later people tried to find the origin of the word in an Aboriginal language, they probably pronounced it wrong and pointed to the wrong species of kangaroo. On drawing a blank with the natives, this gave rise to the belief that the kangaroo was named in error.
The true story is now well known among linguists and starting to permeate more generally, but cute memes about bamboozled explorers tend to survive. And some of the cute stories are true. The Malagasy creature the indri means "look at that!" in Malagasy. The Queensland town Toowoomba may come from the English "swamp".