Cushy is certainly to be found in WW1 literature, and is ascribed a hindustani origin by pte 19022 Frederic Manning in The Middle Parts of Fortune [AKA Her Privates We] first published 1929.

Course that doesn't mean he was right, but he was an educated, somewhat scholarly, man. He was also familiar with the language of the ranks, having served as a private in the Somme area in autumn 1916.