1) Australian English term 2) Australian ranch hand 3) Young ranch hand in Australia 
[trainee] A jackaroo is a young man (feminine equivalent jillaroo) working on a sheep or cattle station, to gain practical experience in the skills needed to become an owner, overseer, manager, etc.{full|date=November 2012} The word originated in Queensland, Australia in the 19th century and is still in use in Australia and New Zealand in t...
• Alt. of Jackeroo • (v. i.) To be a jackaroo; to pass one`s time as a jackaroo.
Jack`a·roo' noun Also Jack`e*roo' [ Jack + kang aroo .] A young man living as an apprentice on a sheep station, or otherwise engaged in acquainting himself with colonial life. [ Colloq., Australia] 
A male station hand (a station is a big farm/grazing property)
a male trainee station manager or station hand (a station is a big farm/grazing property)