What is the origin of "Kris Kringle"?
Webster's Dictionary traces "Kris Kringle" back to the German "Kristkindl" meaning "Christ Child" and indicates the first documented use in English was 1830. According to http://www.etymonline.com/index.php?term=Kriss+Kringle, it entered the English language through the immigration of Pennsylvania Dutch settlers.
This was likely the work of Martin Luther, who condemned the Roman celebration of Saint Nicholas (AKA Santa Claus) on December 25. The Roman celebration was so popular among the common folk that they undermined Luther's prohibition with a touch of syncretism. The Christ Child and Saint Nick (the patron of merchants and children) still share the holiday today!
Santa Claus is a phonetic variant of Saint Nicholas. As the patron saint of merchants and children, the church encouraged the blessing of these two groups during his holiday. Buying gifts is a practical blessing for merchants and giving gifts is a practical blessing on children.
The name Kris Kringle is a recognized substitute for Santa Claus in the United States, but in many other countries they are distinctly separate characters.
I was born and raised in Munchen, Germany. There in the 40''s and 50's we celebrated St. Nicholas Day on the 6th of December. Kriskindle ( Christchild) brought presents o n Christmas Eve.
Early instances of 'Kris [or Kriss] Kringle'
In U.S. book and newspaper databases, the spelling "Kriss Kringle" seems to be slightly older than "Kris Kringle." The earliest mentions are related to a book for children titled Kriss Kringle's Book. A mention of it appears in "Literary Notices," in the New York Herald (November 28, 1842):
KRISS KRINGLE'S BOOK, OR SAINT NICHOLAS' BOOK, FOR ALL GOOD BOYS AND GIRLS. Philadelphia. Thomas, Cowperthwait & Co. This is [a] very neat and beautiful volume of stories, illustrated with numerous cuts, intended, and well adapted for a Christmas present to good boys and girls. Among the stories we notice, "Good Gudule, the Faithful old Nurse," "The Shepperd," "The German Faust," "The Hut in the Wilderness," "Taj Mahal Agrah," and many other "Stories," all with beautiful engravings. We have no hesitation in recommending this as an elegant Christmas present.
And from "Editors' Book Table," in Godey's Lady's Book (December 1842):
Kriss Kringle's Book. Philadelphia: Thomas, Cowperthwaite, & Co., 1843. A new edition of this popular juvenile annual is out, and expectation on the part of the young people beats high. Some thousands of this beautifully ornamented volume, with its gilded covers and coloured picture will find their way into the stockings hung up in chimney corners in anticipation of Kriss Kringle's annual visit. The moral tone of the stories is as commendable as the beauty of the execution.
Mention of the same book appears in "Ticknor's Catalogue of Christmas and New Year's Presents, for 1842" (Boston, 1842) and in an advertisement of "New Useful, and Entertaining Books for the People and Their Children," also in Boston, in an 1842 edition of Nathaniel Hawthorne's Grandfather's Chair: A History for Youth.
The earliest mention that Elephind and Library of Congress uncover for "Kriss Kringle" apart from the book of that name is from "Christmas" in the [Stroudsburg, Pennsylvania] Jeffersonian Republican (December 28, 1842):
Christmas has come and passed; and we believe was never more generously observed before. Notwithstanding the hardness of the times, every body most, seemed to be joyful and merry. The little folks set their plates and hats on Saturday evening, and after passing an unquiet night, got up the next morning to find that St. Nicholas, or Kriss Kringle, as he or she is called, had supplied them most bounteously with all kind of nick-nacks. Their seniors provided themselves with venison, turkeys, geese, chickens, or whatever else good came in their way, and fared sumptuously on Sunday and Monday.
Another early instance is Julia Fletcher, "The Christmas Gift," in The Universalist and Ladies' Repository (April 1843):
Many a heavy heart grew light, and many a heavy purse grew lighter, amid the purchases of that evening. Gifts there were for parents, sisters, brother, and friends, and there too were toys for the little ones who were fast asleep at home, with their stockings hung in the chimney awaiting the visit of Kriss Kringle. Many a young dreamer that night saw their welcome visitor descend the chimney with his wallet of toys, and many a listening ear heard 'the tramping' of his tiny steeds 'upon the roof.' It might have passed for a dream, however, had not the well filled stockings in the morning borne witness that Kriss Kringle had been there.
And from "Poetical Prose," originally published on January 1, 1847 in the St. Louis [Missouri] Reveille, reprinted in Scientific American (January 30, 1847):
"Once a year," and whether cloudy or clear, who feels not, if his blood selfishly steals not, that man is is his brother, and that we ought to love one another,—that is in reason; say once a season! Home and its sweet graces—children's faces; smiles and joy, and old Kriss Kringle; laugh and shout; hark, how they mingle?
The first mention of "Kris Kringle" (with one s in his first name) is in James Rees, "A Christmas Legend" in the Cooper's Clarksburg [Virginia] Register (December 24, 1851), which is also notable for treating Kris Kringle and Santa Claus as different personages:
Their daughter now spoke. 'Mother, I have tied my stocking to that big nail near the fire place, do you think KRIS KINGLE will come down the chimney to-night?'
...
'Then, father, I will take my stocking down, I thought, indeed, mother, I thought that Santa Claus and Kris Kringle loved all those who are good, and have not I been good? I know my lesson, I love you, mother, and my brother dearly, and do whatever I am told.'
A Kringle in Time
Recent examinations of Santa Claus in U.S. folklore identify Kriss Kringle as being a separate entity in past folk descriptions of Saint Nick. Phyllis Siefker, Santa Claus, Last of the Wild Men: The Origins and Evolution of Saint Nicholas, Spanning 50,000 (1997) traces the evolution of the Germanic gift-bearing folktale figure Pelznichol to Bellsnickle, and his gifts from Grisht-kindle ("the word for Bellsnickle's gifts among the Pennsylvania Dutch") to Kriss Kringle (the personification of those gifts and their giver). Siefker writes:
In the 1840s the German gift-giver enjoyed wider exposure when he found his way into print as Kriss Kringle and Bellsnickle. In 1842, Kriss Kringle's Book appeared, followed in 1843 by Bellsnickle's Gift or a Visit from St. Nicholas. In 1845, a popular book, Kriss Kringle's Christmas Tree, was published. The effect of these publications was that by the midd-1840s the German Christmas man had ventured out from his ethnic community to win the hearts of children everywhere.
Alfred Shoemaker, Christmas in Pennsylvania: A Folk-Cultural Study (1959) devotes a fascinating 14-page chapter to the evolution of "Christ-Kindel [the Christ child] to Kriss Kringle." Shoemaker is especially strong in quoting from early sources. Following is a passage about three very early published references to antecedents of "Kris Kringle":
The earliest printed use we have been able to find for the term Christ-kindle" is from the York [Pennsylvania] Gazette of December 23, 1823. In a humorous entry the Society of Bachelors of York announce their intention of "fixing a Krischtkintle Bauhm" which is to say a "Christ-kindle tree." From this time up to 1840 we have been able to locate but three additional instances of its use. John F. Watson in his 1830 Annals of Philadelphia writes "Every father in his turn remembers the excitements of his youth in Belsh-nichel and Christ-kinkle nights." A reporter in the Germantown [Pennsylvania] Telegraph of December 24, 1834, speaking of the anticipation of the child at Christmas, remarked: "How his eye sparkles, and his cheek flushes as he listens to the promises which his glorious friend Chryskingle is to realize." The third instance is from the Gentleman's Magazine of December 1837: "It {Christmas} is a day when 'wee responsibilities' rejoice in 'Christkingle's visit."
This last quotation appears in "Christmas: Addressed to the Philadelphians by a Fellow-Citizen" in The Gentleman's Magazine (December 1837), in which "Christkingle" is credited with filling "vast stockings duly forked up over the chimney the preceding Christmas eve" with "treasures." The 1830 quotation from Watson appears in a chapter of Annals of Philadelphia and Pennsylvania in the Olden Time titled "Sports and Amusements"; I suspect that this is the source of the 1830 occurrence of "Kris Kringle" that Etymology Online cites, as noted in ScotM's answer.
Another account of the evolution of Kriss Kringle appears in Stephen Nissenbaum, The Battle for Christmas (2010). One interesting reference in that book is to an entry from December 24, 1842 in a journal kept by a Morgantown, Pennsylvania shopkeeper named James Morris:
Christmas Eve—a few “belsnickels” or “kriskinckles” were prowling about this evening frightening the women and children, with their uncouth appearance—made up of cast-off garments made particolored with patches, a false face, a shaggy head of tow, or rather wig, falling profusely over the shoulders and finished out by a most patriarchal beard of whatsoever foreign (material} that could possibly be pressed into such service.
An early etymological comment appears in a letter to the editor of the Miners' Journal and Pottsville [Pennsylvania] General Advisor (January 9, 1847) [also cited by Shoemaker]:
In conclusion let me say to you, that I have lately seen a very frequent reference to the Krist Kringle and his frequent visits. Now my dear Sir, I beg leave to say to you, that I am really astonished that a gentleman who possesses so much knowledge of German as I know you do, did not once discover that Krist Kringle is an unwarrantable change of the word Christkindlein, which is one of the beautiful compounds in which the German language abounds, meaning the "Little Child Jesus."
Conclusions
The figure of Kris Kringle has lurked in German American folklore since the early 1800s, but the name "Kris Kringle" has had many antecedents, including Christ-kindel, Christ-kinkle, Krischtkintle, Christkingle, Chryskingle, Grisht-kindle, kriskinckles, Kriss Kringle, and Krist Kringle.
It seems highly probable that "Kris Kringle" owes its preeminence among the various spelling options to the "Kriss Kringle" books of the 1840s and after, since there certainly was no widespread agreement before 1842 as to the spelling of the gift-bringer's name.