[Re. CHARLES] There was no short form until the Americans began to use Chuck, and that form scarcely became popular until the end of the nineteenth century. ... Just where Chuck came from is uncertain. There was an Elizabethan pet-name which had no special connection with a particular name. When Lady Macbeth thus addresses her husband as "chuck," we cannot conclude that his given name was Charles.
--George R. Stewart, American Given Names
And, along the same lines:
CHUCK: English (almost exclusively U.S.) nickname occasionally used as a given name in its own right. It derives from the English term of endearment, itself probably from the Middle English "chukken" to cluck (of imitative origin). It is now often used as a pet form of CHARLES.
--Patrick Hanks & Flavia Hodges, A Dictionary of First Names
So, Chuck apparently does not derive directly from Charles. I suspect that the main reasons it has become attached to Charles is that Charles is the only common, traditional male name in English which begins with the same CH- sound, and the form of Chuck imitates the similar nicknames Jack for John, and Hank for Henry. Both of these clearly do derive from the original name.
This message was edited by the author on November 23, 2015 at 6:46 AM