mack
n.— «Mr. Levine and his client were prisoners in California’s Lompoc Federal Correctional Complex. Like other federal inmates around the country, they found a can of mackerel—the “mack” in prison lingo—was the standard currency.» —“Mackerel Economics in Prison Leads to Appreciation for Oily Fillets” by Justin Scheck Wall Street Journal Oct. 2, 2008. (source: Double-Tongued Dictionary)
Further reading
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