More Jacks than you ever imagined

 

Jack is the name of the hero in many English fairy tales, for example, Jack the Giant Killer, Jack and the Beanstalk and, How Jack Went Out to Seek His Fortune. Another one, Jack the Cunning Thief, is Irish/Scottish. And of course there’s Jack Frost.
Since Jack isn’t a Germanic name, the Grimm Brothers have no Jacks in their collection of tales but “Hans” is used in the same way. 
Nursery rhymes also feature many a Jack — Jack Be Nimble, Jack Sprat,  Little Jack Horner, The House that Jack Built, Jack a Nary, and, Jack and Jill. 
Rhymes that are not so well-known are, “Jack in the pulpit, out and in/Sold his wife for a manikin pin,” and, “Now what do you think of Little Jack Jingle?/Before he was married he used to live single.”
Jack appears in English sayings and proverbs such as, “All work and no play makes Jack a dull boy,” “Every Jack has his Jill,” and, “A good Jack makes a good Jill.” The preceding are from the 17th century. “Jack is as good as his master,” is early 18th century.
Shakespeare employs Jack in the sense of everyman in A Midsummer Night’s Dream (1595-96), when Puck says, “Jack shall have his Jill;/Naught shall go ill” (I, iii, 69-73). In Richard III (1591), Jack is seen as a synonym for low-class: “Since every Jack became a gentleman,/There’s many a gentle person made a Jack” (I, iii, 71-73). 
We encounter Jack in song, as well. A 1916 piece, My Boy Jack, isn’t heard today, but folk songs, Jack Tar, and, Jack in the Green, are still sung. Jack Was Every Inch a Sailor, is a Newfoundland and Labrador folk song which gained wide popularity in the U.S. when Burl Ives recorded it in 1956. A couple of modern songs are, Hit the Road Jack, and, Happy Jack.
And what about the Union Jack?
The correct name for Britain’s flag is, “Union Flag,” so why the Union Jack?  Here, jack echoes nautical usage where it indicates a small flag on the ship’s bow that declares nationality.
Oxford tells us Union Jack is “improper” usage. But it’s probably too late to change things. When I was young, the Union Jack was Canada’s flag. I never heard it called anything other than that.  Most people reading this probably also call Britain’s flag the Union Jack.
An old Celtic folk tale tells us the Jack in Jack-o-Lantern was an aged blacksmith who traded his soul to the devil in exchange for seven years of prosperity. When he died and went to the Underworld, Satan decided he didn’t want him. He turned the smith away and sent him to St. Peter in Heaven. Jack was eating a pumpkin at the time and he snatched a glowing ember from Hell’s fires and put it in the pumpkin so he could carry a lantern to keep evil spirits away in case Satan changed his mind.
Whether we realize it or not, we still use jack-o-lanterns to keep evil spirits at bay.