Japanese Lesson: Slang
There are many Japanese slang words, and many of them are actually meant as insults. I'll be covering some of those as well, as well as the most common words you'll read/hear.
バカ baka: You've probably heard this one before, it's commonly used as fool.
あほう ahou: This is even ruder than using baka, both meaning a foolish person, but this one being more harsh.
あまい amai: This can be used to descibe someone who is gullible. I'm not too sure on the kanji, but the reading is the same for the adjective 'sweet'
ばいと baito: Slang term for a part-time job, or someone who is working part-time
美人（びじん）bijin: Used to describe an attractive woman
イケメン ikemen: Used to describe a handsome man
ダメ dame: Term used like 'bad' or 'useless'
ファイト faito: Used as a sort of 'good luck!' word, similar to 頑張って
けいた keita: Cell-Phone
マジ maji: Used as 'really??'
めちゃ mecha: An adverb that means 'extremely'
Many of these words technically weren't 'slang', but rather words that you use with people you are familiar with. However, they aren't the words you would really say to your teacher or boss.