A term applied to a particularly lol-worthy bout of laughing, or a particularly enjoyable time or event.
Combination of the words rofl; meaning to roll on the floor laughing, and fest; denoting a gathering or activity specified by a particular activity. In this case; laughing.
Kid #1: Dude, do you remember Charlie's party?!
Kid #2: Do I ever! It was a right rofl-fest!