Yes, THRASHES is a Scrabble word!

Letter values in Scrabble US: T 1, H 4, R 1, A 1, S 1, H 4, E 1, S 1.

Is it a valid word?

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

give a thrashing to; beat hard
move or stir about violently
dance the slam dance
beat so fast that (the heart's) output starts dropping until (it) does not manage to pump out blood at all
move data into and out of core rather than performing useful computation
beat the seeds out of a grain
beat thoroughly and conclusively in a competition or fight

Also: thresh about, thrash about, slam dance.

Not playable in Scrabble US: JACTITATE.