The word sideburn used to be spelled burnside, but then in the 1870s some people erroneously figured that the former spelling had to be the correct one because of the hair being on the sides of your face (and through confusion with another term, sidewhisker), so we got stuck with the new word. However, neither sides nor burns have anything to do with the it, which was named after a Civil War general named Ambrose Burnside, who was known for his rather magnificent facial hair (pictured). Originally, the term only referred to the particular style of beard that left the chin clean-shaven while the sides turned into a moustache, but as that grew less popular it came to increasingly be associated with hair just on the sides of the face. The word sideburn peaked in usage in 2003 after a massive increase in the early- and mid-twentieth century.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.