Twenty-One is first recorded in the 18th century under the name Vingt-Un or Vingt-et-Un, showing it to be of French provenance. In Britain and Ameria it was played under its French name throughout the 19th century, though at some time in England it was pronounced in such a way as to be occasionally spelt Van John.
Thought to derive from another popular card game called Chemin de Fer, also originating in French casinos, Blackjack was first known as Vingt-et-Un, which simply translates to Twenty-One.