Meaning of Kusa(草)
草 is a Japanese internet slang and means “lol”.
This word is born at the most famous internet forum.
In the interest forum, they used “(藁)” as “lol” long time ago. That pronunciation is “wara” (in Japanese “warau” means laugh or smile.) but this kanji means straw not laugh or smile.
After “(藁)” they started to use “w” because “w” is the first character of “wara. Yes, it’s just omitted.
Sometimes it’s used like this “wwww”. The more “w” there are, it means more laughable.
Recently “w” is becoming common word, but if you want to use this you should read the atmosphere. because there are still internet slang. More formal word than “w” is “(笑)” so it’s safety to use this . (But I think most safety way is don’t use both “w” and “(笑)” till someone use.)
Then, finaly I can explain about “草”! (This kanji means grass.) Origin of “草” is “w”. Why? Look at this.
Does it looks like grass? Look as a picture.
Before we use “草” we used “草はえる”. It means “grow grass” and we still use “草はえる” too on internet.
[addition: Dec 8th, 2020] This word is a still popular slang. So, you can still use this.
Example Conversation (on chat)
A: Listen! Tanaka overslept again!
B: Really? Again? Hilarious!w
A: And he came to office with his mother!
B: 草. We can’t avoid to laugh a lot.
B: マジで？ また？ うけるんだけどw
[Addition : Aug 14th, 2020]
This expression “大草原不可避” is already out of date.
These days, most popular way to use “草” is maybe “～すぎて草(～sugite kusa)”
“～Sugite” means “too ~”.
If you want use this, use like this.
あほすぎて草(aho sugite kusa)
あほ(aho) means stupid, so it means “It’s funny because it’s so stupid”.