ワロタ
Warota
わろた
Evolved from 'waratta' (laughed) → 'warota' — a classic internet slang from 2ch (now 5ch) forums in the 2000s. 'Kuso warota' (laughed my ass off) is a staple expression. While 草 (kusa) has largely replaced it in modern internet culture, it's still understood by anyone familiar with Japanese forum culture. A similar term is ワロス (warosu).
Examples
あのコピペ何回見てもワロタ
That copypasta is funny no matter how many times I see it — warota.
About a funny post on an internet forum
クソワロタwwwww
Kuso warota lmaooo wwwww
Reacting to something funny on social media
電車で隣の人のスマホ見えちゃってワロタ
I accidentally glanced at the phone of the person sitting next to me on the train — warota.
不覚にもワロタ
I laughed in spite of myself — warota.