The word kangaroo derives from the Guugu Yimidhirr (an Australian Aboriginal language) word gangurru, referring to a grey kangaroo. The name was first recorded on 4 August 1770, by Lieutenant (later Captain) Cook on the banks of the Endeavour River at the site of modern Cooktown, when HM Bark Endeavour was beached for almost 7 weeks to repair damage sustained on the Great Barrier Reef.
Kangaroo soon became adopted into standard English where it has come to mean any member of the family of kangaroos and wallabies. The belief that it means "I don't understand" or "I don't know" is a popular myth that is also applied to many other Aboriginal-sounding Australian words. Male kangaroos are called bucks, boomers or jacks; females are does, flyers, or jills and the young are joeys. The collective noun for kangaroos is a mob, troop, or court. Kangaroos are sometimes coloquially referred to as 'roos.
Heard it from an Aussie