He does not know a libel from a lupine. In Latin: “Ignorat quid distent ara lupinis,” “He does not know good money from a counter, or a hawk from a handsaw.” The Romans called counters lupines or beans. A libel was a small silver coin the tenth part of a denarius = the as.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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