Yes, BUTTOCKS is a Scrabble word!

Letter values in Scrabble US: B 3, U 1, T 1, T 1, O 1, C 3, K 5, S 1.

Is it a valid word?

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

the fleshy part of the human body that you sit on
either of the two large fleshy masses of muscular tissue that form the human rump

Also: hind end, rear end, tail end.

Not playable in Scrabble US: TOOSHIE.