In Thomas Paine's The American Crisis, while ridiculing Sir William Howe and his recently obtained knighthood, said:
As a proper preliminary towards the arrangement of your funeral honors, we readily admit your new rank of knighthood. The title is perfectly in character, and is your own, more by merit than creation. There are knights of various orders from the knight of the windmill to the knight of the post. The former is your patron for exploits, and the latter will assist you in settling your accounts.
A knight of the post is a professional false witness, but I can't find what a knight of the windmill is.
The phrase is a likely reference to the book, Don Quixote. Thomas Paine is familiar with the book and uses the imagery in the "Rights of Man" to attack Edmund Burke:
In the rhapsody of his imagination, he has discovered a world of windmills, and his sorrows are, that there are no Quixotes to attack them.”
Don Quixote follows the adventures of Alonso Quixano, an hidalgo who reads so many chivalric novels that he decides to set out to revive chivalry, under the name Don Quixote. He recruits a simple farmer, Sancho Panza, as his squire, who often employs a unique, earthly wit in dealing with Don Quixote's rhetorical orations on antiquated knighthood.
From this book comes the phrase "tilting at windmills."
a professional false witness of 15th to 17th century England.
In drawing a contrast between the romantic and chivalrous Knights of the Windmill, and the completely despicable professional liars and oath-breakers-for-hire termed Knights to the Post, Paine makes the point that knighthood in and of itself bestows no mark of integrity or honesty; but that such qualities are of the man himself and not the title.