大草原

Of 大(だい) (dai, “great”) + 草(そう)原(げん) (sōgen, “grassland”). 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.

• 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

   大(だい)草(そう)原(げん)の小(ちい)さな家(いえ)

   Daisōgen no chīsana ie

   Little House on the Prairie

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

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