Upon search, I found out that sideburns has evolved from burnsides, named after the Civil War veteran and Rhode Island senator Ambrose Burnside. See here
But surely sideburns existed before him, so what where they called then? Simply beard on cheeks/sides of face?
Best Answer
The style shown in your link was called...
mutton-chop beard
also
mutton-chop whiskers
This only refers to the luxuriant type of facial hair style as pictured in your link. It is still in use today.
The phrase seems to go back to approximately 1845, according to N grams.
However, some probably just referred to them as
whiskers
Notice that in the descriptions in Prints of English Heads, "whiskers" is listed separately from styles of beards.
In the next transcript, from the The Conspiracy Trial for the Murder of the President: And the ..., Volume 2
So it appears that what we call side-burns now was originally called either side-whiskers,or mutton-chop whiskers (also mutton-chop beard) , depending on the style. What I found confusing in the search was that "whiskers" was also used to describe a mustache.
Beards have come and gone in a variety of styles for men, but the "sideburn" was probably only seen after 1800, and then only as part of a beard.