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Definition of black cab in US English:
black cab
noun
A black taxicab; (now usually) (British) a type of taxi, originally and usually (but now not always) black, which may be hailed in the street and can accommodate up to five passengers in the rear, separated from the driver's seat by a partition (officially called Hackney cab).
Origin
Early 20th century; earliest use found in Cosmopolitan.
