More info about the name "Jack"

Jack is a medieval diminutive of John. John originates in Hebrew language and means "God is merciful". It has been one of the most popular masculine given names over centuries, and in different spellings all over the world. Throughout history it was borne by numerous kings, princes, emperors, popes and saints, the most famous being John the Baptist and John the Apostle. More recently, the famous bearers include John F. Kennedy and John Lennon.