大草原

• IPA^(key): [da̠iso̞ːɡẽ̞ɴ]

大(だい)草(そう)原(げん) • (daisōgen) 

1. grassland, prairie, savanna

   Synonyms: 草原, 草地, サバンナ, プレーリー, ステップ

2. (Internet slang) LOL, LMAO; used to indicate amusement or laughter
   • 2018 February 18, Hikaru Yoza, quoting Twitter user @Ryusei_Rose, “‘Warau’ no hyōgen ga shinka shiteiru gen [The case of evolving expressions for ‘laugh’]”, in BuzzFeed‎:

     さすがに草(くさ)生(は)えるわ www 大(だい)草(そう)原(げん)不(ふ)回(かい)避(ひ)

     sasuga ni kusa haeru wa www daisōgen fukaihi

     as expected, laughs lol, can’t stop lmao

The Internet slang use developed as an exaggeration of 草(くさ) (kusa, “LOL”, literally “grass”), which in turn developed from ww used to indicate laughter in online texts.

• 草(くさ) (kusa, “LOL”)
• 草(くさ)生(は)える (kusa haeru, “laugh; be funny”)