Hamlet says to the Ghost, “Art thou there, Truepenny?” Then to his comrades, “You hear this fellow in the cellarage?” (i. 5). And again, “Well said, old mole; canst work?” Truepenny means carth-borer or mole (Greek, trupanon, trupao, to bore or perforate), an excellent word to apply to a ghost “boring through the cellarage” to get to the place of purgatory before cock-crow. Miners use the word for a run of metal or metallic earth, which indicates the presence and direction of a lode.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894