Ambrose Burnside was a Civil War general (Union Army) with impressive facial hair.
After he appeared in parades in Washington, DC, his long bushy side-whiskers became fashionable.
Those whiskers were first known as burnsides; the word later grew into sideburns.
Definition courtesy of Merriam-Webster, as part of their Top 10 Words from People’s Names, which can be viewed by clicking here. Lynch, maverick, boycott, and other cool words can be found there, as well.