Old English myrgð "joy, pleasure, eternal bliss, salvation" (original senses now obsolete), from Proto-Germanic *murgitha (source also of Middle Dutch merchte), noun of quality from *murgjo- (see merry; also see -th (2)). By early 13c. as "expressions or manifestations of happiness, rejoicing;" by mid-14c. as "state or feeling of merriment, jollity, hilarity." Mirthquake "entertainment that excites convulsive laughter" first attested 1928, in reference to Harold Lloyd movies.
I HAVE always preferred chearfulness to mirth. The latter, I consider as an act, the former as an habit of the mind. Mirth is short and transient, chearfulness fixed and permanent. Those are often raised into the greatest transports of mirth, who are subject to the greatest depressions of melancholy: on the contrary, chearfulness, though it does not give the mind such an exquisite gladness, prevents us from falling into any depths of sorrow. Mirth is like a flash of lightning, that breaks through a gloom of clouds, and glitters for a moment; chearfulness keeps up a kind of day-light in the mind, and fills it with a steady and perpetual serenity. [Addison, "Spectator," May 17, 1712]