Usage examples for jacks:

• There were a great many of the passengers around the table, so we played on the square until everybody went to bed and left us alone; then I ran him up three large jacks and gave my partner three queens, and guarded both hands so that nothing could drop in. "Forty Years a Gambler on the Mississippi" – George H. Devol

• J. Jacks 15 W. Va. "In and Out of Rebel Prisons" – Lieut. A. [Alonzo] Cooper

• John Jacks the M. P.? "The Crown of Life" – George Gissing

• He was quick and accurate, finally scorning to shoot at a stationary mark and often riding miles to get to the valley level where there were rabbits and " Jacks that he occasionally bowled over on the run. "The Ridin' Kid from Powder River" – Henry Herbert Knibbs