[ei] in lingua. He is bribed to silence; he has a coin (marked with a bull's head) on his tongue. Adalardus, in Statutis Abbatiæ Corbeiensis (bk. i. c. 8), seems to refer to the bos as a coin. “Boves et reliquam pecuniam habeat ... unde et ipse et omnis familia ejus vivere possit ” (i.e. plenty of gold and silver ...). Plautus, however, distinctly says (Persa, ii. 5, 16), “Boves bini hic sunt in crumena ” (Two bulls in a purse.) The Greeks had the phrase bouz epi glwtthz. Servius tells us that even the Romans had a coin with a bull stamped on it. (See Pliny, 18, 3.) Presuming that there was no such coin, there cannot be a doubt that the word Bos was used as the equivalent of the price of an ox.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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