This is the commonhappening of someone laughing loudly and exuberantly (in other words scandelously) for a LONG period of time (without the use of laughing gas) when neither the person laughing or the observer(s) know what caused the laughing in the first place.
(Laff a meh tie tuss) n. 1. A rare disorder that exudes uncontrollable laughing especially during non-laughable situations. 2. The inability to control ones laughter.
Dick "Dude.. That cop just questioned us for a murder and fucking Cooper is laughing his ass off."
Jane " Yeah his laughametitis is gonna get us sent to jail!"