Ambrose Burnside was the namesake of the word sideburn. This portrait was taken circa 1860 by Matthew Brady, the father of photojournalism.
The term sideburns is a 19th-century corruption of the word burnsides, named after American Civil War general Ambrose Burnside, a relatively unremarkable soldier known for his unusual facial hairstyle that connected his thick beard by way of a mustache, but left his chin clean-shaven.
Source: The Adventure of English: The Biography of a Language by Melvyn Bragg