It was for American Civil War General Ambrose Everett Burnside, that the term sideburns was derived. Originally, he word burnsides was coined to describe this style. The syllables were later reversed to give sideburns.
Of course, aficionados of facial adornment realize that this particular style of beard already had a name before the Americans came up with the term sideburns in honor of General Burnside. In England, the word mutton chops was first used in 1865 to describe a facial adornment that starts off narrow at the top in front of the ears and widen across the cheek and jawline to the corner of the mouth. When the mutton chops are connected across the face with a mustache, or perhaps described as a full beard with the chin clean shaven, they are called friendly mutton chops. General Burnside has a fine example of friendly mutton chops that became even more distinguished in his later years.