This will be a series that will explain Japanese words often used in Anime. I'm learning Japanese so this will keep me on my toes and force me to go on! I will type it in kanji, hiragana/katakana and romaji each time.
悪い わるい Warui (pronounced warooy) - bad, my bad, sorry, oops.
A derived expression:
atama ga warui (atama - head) - slow, dumb, stupid (bad head)