Living in Japan, you start to realize that Japanese people don't speak in textbook Japanese. Like anywhere in the world, they love to use slang. Here's a list of slang I often heard (and used) in Japan.
1. Yabai, ヤバイ
This is probably the most common slang you will here. I heard it on a daily basis. It's usually used in surprised situation whether the situation is good or bad. You tell your friend you failed an exam, they might say "Yabai!". You see your friend making out with someone in the club you can say "Yabai!". It's a great word that can be used in many different situations.
2. Kimoi, キモイ
Kimoi is a shortened version of 'kimochi ga warui' which mean 'feels bad'. You can say kimoi when you touch something that feels gross or someone does something gross. If you ever find yourself in a gross situation make sure to use this word and impress all your Japanese friends.
3. Charai, ちゃらい
Charai is a word to describe guys that are "players". Usually there is a certain look that comes to mind when talking about charai guys and has a negative connotation. If your friend tells you a guy is charai you should probably stay away from them.
4. Mecha, めちゃ
When learning Japanese in a textbook they will teach you to use 'totemo' as the adverb very. However, most young Japanese people use the word mecha instead. So when you hear the word mecha now you won't be so confused!
5. Maji, マジ
This is another one you would hear on a daily basis in Japan. Maji is used whenever someone says something surprising. It translates as "really?" Or "are you serious?".
These are a few Japanese slang words that you will often here in Japan. Try using these words with your Japanese friends and you will definitely impress them!