Before the 15th century, European playing cards used to show a King, a
Cavalier on a horse and a simple foot soldier or male servant. Thus,
the social ranking of the characters displayed the value of the cards.
For unknown reasons, French card makers replaced the cavalier with the
queen, but the lowest of the court cards with the servant remained
In England, those court cards were called "King", "Queen", and
"Knave", with knave (related to the German "Knabe" = boy) being a term
for a male servant, a meaning now unusual in modern English.
In old English, "Jack" was a men's name, but it was also used as a
terms for "commom man". This might be related with the French mock
expression for peasants, "Jacques", which was widely used because it
was considered a typical name of common country folks.
Obviously derived from this depreciatory meaning, "Jack" became a term
for small, worthless things. For example, the smallest English coin,
the farthing, was colloquially called a jack and chips used in
gambling were called jacks because of their worthlessness.
Because of that, in the game of All Fours, "Jack" became the name of
the point awarded for winning a trick containing the Knave of trumps.
It was therefore also applied to the Knave of trumps itself in that
game. Later it also meant any of the Knaves, which fits very good with
the images on the cards.
In the mid-19th century, the Jacks cards were still referred to as
Knaves. When it was felt necessary to label the corners of each card
with an index to indicate its value, the makers naturally picked the
first initial of two court cards giving the indices K (for the King),
Q (for the Queen) and the first 2 initals of Knave giving Kn.
Obviously this was confusing, so by using an alternative term for a
common man which was also associated with the Knave card, Jack, we end
up with court indices being an unambiguous K, Q, J and the Jacks
finally called Jacks.
The Most Frequently Asked Questions About Playing-Cards, Part 2: Why
are Jacks called Jacks? By Daphne Tregear, 2001
Take Our Word For It, Issue 97
Search terms used:
jacks "playing cards"
"playing cards" "called jacks"
Hope this answers your question!