Supposedly so named for the Irish people driving them, the Irish people getting arrested and thrown into them, and the padding inside to prevent injury.
From Wikipedia:
Paddywagon and Black Maria are slang terms for either a police car, or a police vehicle used to transport large groups of people who have been arrested.
The word paddywagon is of American origin. The precise origin of the term is uncertain and disputed, though its use dates back to at least the 1930s.[1] There are at least three theories as how the phrase originated.[2]
The most prevalent theory is based on the term "Paddy" (a common Irish shortening of Patrick), which was used (sometimes as slang) to refer to Irish people.[3] Irishmen made up a large percentage of the officers of early police forces in many American cities. Thus, this theory suggests that the concentration of Irish in the police forces led to the term "paddywagon" being used to describe the vehicles driven by police. An alternative theory is similarly based on the term "Paddy" but states that the term arose due to the number of Irish being arrested for spirited behavior and hauled off in the vehicles.[4] The final theory holds that the name originates from the padding used on the inside of police horse-drawn carriages to prevent injury.[5]
These vehicles were usually painted black. Archaically in the United States, Ireland and the United Kingdom, a police wagon was also sometimes called a Black Maria (using the archaic pronunciation i.e Mariah). The origin of this term is equally uncertain. The OED lists the first usage as the Boston Evening Traveller from 1847 which mentions them as a new type of wagon. Brewer's Dictionary of Phrase and Fable suggests the name came from Maria Lee, a large and fearsome black madam who the police would call on for help with difficult prisoners. The term is still used today in parts of Britain for the vehicle that transports prisoners from gaol to court, appearing in the song "Guns of Brixton" by The Clash. Frequently, screened-in buses are also used for the same purpose. [1]
The term is often used in some areas of Australia, specifically New South Wales[2] and Queensland[3] to refer to a general duties vehicle with a prisoner cage on the back. Australian paddywagons are typically based on small utility vehicles such as the Holden Rodeo or Toyota Hilux. In other areas of Australia the term Divisional Van (or Divvy Van for short) is more common
