Green’s Dictionary of Slang

expenses.

1865		Leaves from Diary of Celebrated Burglar 42/1: Each ‘mob’ said that their own particular ‘mob’ would make double their X’s at the least, and offered to bet on it.
1880		Bulletin (Sydney) 26 June 7/4: Darrell [...] was induced to remain a week in Brisbane after the termination of his engagement, the young man undertaking to pay all his x’s.
1909		(ref. to 1860) in J. Ware Passing Eng. of the Victorian Era.