ROFL

Сокращение от rolling on the floor laughing, по русски - валяюсь на полу от смеха. Русскоязычный аналог -  "пацтолом". 



That's so funny! ROFL!



Вариации: 



ROTFL (Rolling On The Floor Laughing)



ROFLMAO (Rolling On the Floor Laughing My Ass Off)



ROFLMAOSTC (Rolling On The Floor Laughing My Ass Off Scaring The Cat).