English vs. British
This expression is probably more than four centuries old, and controversy remains over whether it was originally rule the roost or the roast. The former refers to the rooster who oversees the hens. The latter alludes to the master of the house who presides over carving and serving roast meat at the family table. Americans use the first expression to mean "in charge," while Britons prefer the second. To make matters worse, "roost" was once pronounced and even spelled "roast," and vice versa.
A cry of derision in one country may be an acclamation in another. In England, a "guy" is a ridiculous person of eccentric appearance. It is derived from Guy Fawkes, leader of a plot to blow up Parliament in 1605. Grotesque effigies of Fawkes are carried through the streets of England and burned in bonfires on Guy Fawkes Day, Nov. 5. In America, a guy can be a fellow, a joe, or even a dude.
SOURCE: 'The Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson.