The Palace of Westminster was an earlier version of the Houses of Parliament and some cynics have pointed out that Guy Fawkes was the only man to go to Parliament with the intention of keeping a promise. Each Guy Fawkes Day, weird-looking effigies of Mr. Fawkes were created and burned. By the early 19th century, the term Guy was used to refer to anyone looking weird or strange, as those effigies did. From there, guy came to mean any poor or shabbily dressed person. The capitalization of the first letter of Guy is of note there.
Later quotations have the entire word in lower case.
One might wonder if common use of the first name Guy might have had some influence on American usage of the more general term, but the name Guy has not been common at all in the U.S., so that notion holds no water. Some suggest that the Romans appropriated the word from Etruscan, an extinct language spoken in ancient times in what is now Italy. Autumn entered English in the 14th century and by the 16th century had supplanted the Old English form, harvest , so named for obvious reasons. Fall is a common synonym in the U.S., and it arose in the 16th century, as well. It often appeared in the form fall of the leaf , referring, of course, to the shedding of leaves by deciduous trees.