Brewer's Dictionary of Phrase and Fable suggests the police wagon was named after a fearsome lady called Maria Lee who kept a boardinghouse in Boston in the 1820s.
It was said the police would call her in to help with unruly prisoners.
Which sounds unlikely when they could simply whack them with a big stick.
The World Wide Words site on the internet says the first reference to a Black Maria vehicle in Boston was in 1847, some years after the lady's fame.
But it had been used earlier in New York (1835) and Philadelphia (1844) where they would have been unaware of the said lady.
A likelier connection could be a famous race horse named Black Maria whose greatest victory was a 20 mile race run in New York in 1832.
Three years later, police vehicles in the city became known as Black Marias.
