While nicknames are a shortened form of the original word, the nickname for John is an exception. Jack is a nickname for John. The name is not a shortened version and, aside from the letter J, is decidedly different. The reason stems from the Bible, as well as an influence from John F. Kennedy.
Homework help: Jack and John as peasant names
Historical use of John and Jack provides one explanation for why the names intertwine. Dating back to 1200, according to Mental Floss, the name Jack was often a name for peasants. When an individual peasant’s name was unknown, Jack was a label given that person.
John had a similar background in England. People called English commoners John. The term John Doe developed to refer to someone of unknown identity. As both John and Jack referred to peasants, the two names were often interchangeable. That is one explanation for why Jack is a nickname for John.
Norman explanation for the nickname Jack
The way the Normans addressed people also explains why Jack is today a nickname for John. When Norman people made a diminutive, they added -kin to the name. Their form of John was Jen. The name Jen became Jenkin, which over time developed into Jack.
John F. Kennedy and the name Jack
President John F. Kennedy helped solidify Jack as a nickname for John. Jack was the nickname of the 35th president of the United States as a child. Both family and friends called John Fitzgerald Jack, even as a blue-eyed baby. As an adult and politician, his initials, JFK, became his nickname instead.
Kennedy is a well-known historical figure, making his childhood nickname common knowledge. Given his popularity, people may have begun to associate John and Jack together, even if they did not previously know the two names were interchangeable. John F. Kennedy is famous for his time in office, his Pulitzer Prize win and for the way he died.
His death is part of what has made his name well-known today. Kennedy was assassinated in 1963. Details of the events after his death still arouse discussion today. Police arrested Lee Harvey Oswald for the crime, and two days later Jack Ruby killed Oswald.
The United States House Select Committee on Assassinations (HSCA) found the police investigation to be a flawed venture. The HSCA concluded that the death was likely a conspiracy. Even today, conspiracy theories about JFK are topics of discussion. The tragedy and complexity of his death make him a well-known historical figure, and many people learning more about JFK's death discovered that John F. Kennedy’s nickname was Jack.
The origins of Jack as a nickname for John date back to ancient times. English people called commoners both John and Jack. The names became interchangeable. John F. Kennedy’s childhood nickname of Jack also solidified use of the name.