Yes, BURNSIDES is a Scrabble word!

Letter values in Scrabble US: B 3, U 1, R 1, N 1, S 1, I 1, D 2, E 1, S 1.

Is it a valid word?

BURNSIDES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

United States general in the American Civil War who was defeated by Robert E. Lee at the Battle of Fredericksburg (1824-1881)
facial hair that has grown down the side of a man's face in front of the ears (especially when the rest of the beard is shaved off)

Also: a. e. burnside, ambrose everett burnside, mutton chop, side-whiskers.

Not playable in Scrabble US: BURNSIDE, SIDEBURN.