The word "kangaroo" is believed to have come from the Aboriginal word gangurru, a Guugu Yimidhirr word referring to the Grey Kangaroo. Captain James Cook's botanist, Sir Joseph Banks, first recorded the word as "kangaru" when the Endeavour was damaged on the Great Barrier Reef near modern-day Cooktown, and required the crew to stay on the mainland for almost 7 weeks repairing their ship. This gave Banks ample time to make copious notes on the fauna and flora, and to be certain of the Aboriginal word for kangaroo, even though he had trouble translating it into written form.
It could be that "Kangaroo" is the name of a specific type of kangaroo in one Australian aboriginal language. Europeans applied the name to all similar creatures.
There is no other stage between joey and adult kangaroo.
A kangaroo joey weighs about 0.5 grams at birth.
When first born, a Red kangaroo's joey is barely 2cm long.
No a joey is a baby kangaroo
joey
A joey
a juvenile kangaroo
No, the word "joey" is not capitalized when referring to a baby kangaroo. It is treated like a common noun.
joey
A kangaroo's offspring.
Kangaroo
joey