Another term derived from "LOL", or "Laugh Out Loud". This is a mashup between Lol, and lullabye. Typically, when somebody says a lollabai, the audience laughs so hard that they run out of breath and consequentially pass out.
Lulz of this magnitude can be rare, but not to be mistaken for the Lolz of Jericho (See Lolz of Jericho).