A sixpence. Called testone (teste, a head) because it was stamped on one side with the head of the reigning sovereign. Similarly, the head canopy of a bed is called its tester (Italian, testa; French, teste, tête). Copstick in Dutch means the same thing. Worth 12d. in the reign of Henry VIII., but 6d. in the reign of Elizabeth.
“Hold, there's a tester for thee.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894