I kept seeing it in YouTube comments and in other places, so I finally decided to look it up in Japanese instead of trying English, and it ends up that “grass” = “lol”?
Basically, in online chat & comments, the Japanese “lol” is often written as (笑), sometimes without parenthesis, sometimes even in just kana at the end of the sentence. This gets shortened even further to “w”. If any of you have ever seen any nicovideo stuff, you’ve seen this fly across the screen:
All the w’s look like grass on a lawn, thus 草 meaning something similar to 爆笑, or LMAO.
Once I saw the explanation, it made so much more sense. lol
This is just a guess, but I imagine actually saying this verbally would be similar to saying “lol” or “l.m.a.o.” in actual speech.
Here’s the article: