Green’s Dictionary of Slang

tester n.1

also teaster
[Fr. teston, a silver coin struck at Milan by Duke Galeazzo Sforza (1468–76). It had his own head on it, as did similar testons coined by Louis XII (r.1498–1515) and his successor François I (r.1515–47) of France and by Henry VIII (r.1509–47) of England ]

1. [late 16C–1920s] (also teaster, teston, testone, texter) sixpence.

2. [mid-19C–1940s] (Aus.) 25 strokes of the lash [reflects the association of numbers of lashes with denominations of coins].