If you are bare-assed, you have no clothes on — especially nothing covering your bottom. It is a rough, informal American way to say naked. You might hear it in films or between friends who swear a lot. You would not write it in a school essay, and you would not say it to a teacher or in a shop. British English has a similar word, bare-arsed, with arse instead of ass. Though actually, people also stretch it for a blunt picture, not only for true nakedness, because the word is meant to sound a bit shocking. It is about being uncovered, and it is rude on purpose.