Every year Japanese young generation produces new slang words. Those words are created mostly by Junior high and High school kids and some of them are spread out all across Japan in a short period of time. Most of these new slang words go out of fashion but some of them have been used for a long time. Let me tell you those popular Japanese slang words. However, like the slang words in your country, they are basically not used in a formal occasion. I recommend not to use them but to know these words is very important to understand real Japanese conversation.
This slang word appeared in mid 80’s and it is still used by young kids and even adults. The pronunciation of “Cho” is similar as “choke” without “ke”. Meaning of this word is “very”, “super”, or “extraordinary”. So it can be used with almost all adjectives. You might know a Japanese word of “Kawaii”, which means “pretty” or “cute”. When high school girls find cute clothes or shoes at a store, I bet they would scream “Cho-Kawaii!!”.
Originally, This word was used by Japanese mafia (Yakuza) but in 80’s, young generation started using in their daily conversation with their friends. At that time, this slang word describes something dangerous or weird. So it was negative term. However, in the late 90’s, similar as the US slang term ”awesome” or “sick”, the word recently has been used as the same meaning as “very good” or “splendid”. It is frequently used with the slang word “Cho”. “Cho Yabai” now means “super cool”.
I guess some slang words in your country are made just shortening the original words. The slang word “Kimoi” is an example of such slang words. Its original word is “Kimochi-Warui”. “Kimochi” means “feeling” and “Warui” means “bad”. So “Kimochi-Warui” means “gross”, “yucky” or “creepy”. “Kimoi” is relatively new slang but has been spread widely in a short period of time. The word has same meaning as “Kimochi-Warui” and now used very heavily by young kids. Since it is not formal word, parents and Japanese teachers usually frown when they the kids use it.
This slang word means “annoying” or “uncomfortable”. Same as “Kimoi”, it is kind of new slang word but you could hear it frequently if you are in Japanese Junior High or High school class room. This word is sometimes used for cursing other person like teachers, parents or school mates.
If you are with a group of Japanese persons and one of them hear something from the other guy and is surprised, she/he would say “Majide?” or “Maji?”. This is completely same meaning of “Really?”. When the term is used in an affirmative sentence, it means “for real”. You may want to use the slang word in a casual situation. Try saying “Majide Arigato” to your Japanese friend when you want to say “thank you very much”. I guess the friend would laugh out loud!