The Collaborative International Dictionary of English v.0.48:

Jackaroo \Jack`a*roo"\, n. Also Jackeroo \Jack`e*roo"\ [Jack +
   kangaroo.]
   A young man living as an apprentice on a sheep station, or
   otherwise engaged in acquainting himself with colonial life.
   [Colloq., Australia]
   [Webster 1913 Suppl.]