"short side-whiskers, part of a man's beard which grows on the cheeks," 1880, American English, an alteration of burnsides (q.v).
Entries linking to sideburns
burnsides n.
style of facial hair consisting of side whiskers and a mustache (but clean-shaven chin), 1875 (singular; plural form from 1878; many early uses are in college and university magazines), a reference to U.S. Army Gen. Ambrose E. Burnside (1824-1881) of Civil War fame, who wore them and inspired the style. Compare sideburns.