Yes, thrashes is a Scrabble word!

Verb

1. Give a thrashing to; beat hard

2. Move or stir about violently

3. Dance the slam dance

4. Beat so fast that (the heart's) output starts dropping until (it) does not manage to pump out blood at all

5. Move data into and out of core rather than performing useful computation

6. Beat the seeds out of a grain

7. Beat thoroughly and conclusively in a competition or fight

Noun

1. A swimming kick used while treading water