Jacks

Playing cards were invented in Imperial China. They were found in China as early as the 9th century during the Tang Dynasty (618–907)...

The four suits now used in most of the world — spades, hearts, diamonds, and clubs — originated in France in about 1480...

Also in the 15th century, Europeans changed the court cards to represent European royalty and attendants, originally "king", "chevalier" (knight), and "knave". The original meaning of knave was male child (cf German Knabe), so in this context the character could represent the "prince", son to the King and Queen...

Before this time, the lowest court card in an English deck was officially termed the Knave, but its abbreviation ("Kn") was too similar to the King ("K") and thus this term did not adapt well to indices. However, from the 17th century the Knave had often been termed the Jack, a term borrowed from the English Renaissance card game All Fours where the Knave of trumps has this name. All Fours was considered a game of the lower classes, so the use of the term Jack at one time was considered vulgar. The use of indices, however, encouraged a formal change from Knave to Jack in English language decks.
Wikipedia.

Alphabet site J