Lolki Lawlingontheflorollin is the mythical being of mischief in Troll mythology, a son of the giants Fatty and Lawley, and foster-brother of Trolldin. He is described as the "contriver of all fraud". He mixed freely with the gods for a long time, even becoming Trolldin's blood brother.
(Taken from Encyclopedia Dramatica)
Lolski is an interjection used originally by internet trolls, but it's use has become common throughout social websites and messengers. It is usually exclaimed after a bout of hilarity, most commonly occurring in video games and message boards.
David throws a banana peel in front of Adam, who trips and face-plants; David's responds with a definitive "Lolski!"