Wallaby comes from Dharug walaba / wallabi (small kangaroo); adopted into modern English as wallaby, a small or medium-sized kangaroo-like marsupial. In current use it is a noun meaning "a small to medium-sized marsupial native to Australia and nearby islands, similar to a kangaroo but generally smaller". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The history is mainly one of transmission into English, with the older form and sense carried through borrowing or adaptation. For dictionary purposes, the important point is the continuity between the older form, the route into English, and the present lexical sense.