Etymology
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mirth (n.)

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]
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mirthless (adj.)

"joyless, without mirth, unhappy," late 14c., from mirth + -less. Related: Mirthlessly.

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mirthful (adj.)

early 14c., "delightful," from mirth + -ful. Related: Mirthfully; mirthfulness.

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*mregh-u- 
Proto-Indo-European root meaning "short."

It forms all or part of: abbreviate; abbreviation; abridge; amphibrach; brace; bracelet; brachio-; brachiopod; brachiosaurus; brachy-; brassiere; breviary; brevity; brief; brumal; brume; embrace; merry; mirth; pretzel; vambrace.

It is the hypothetical source of/evidence for its existence is provided by: Greek brakhys "short;" Latin brevis "short, low, little, shallow;" Old Church Slavonic bruzeja "shallow places, shoals;" Gothic gamaurgjan "to shorten."
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drollery (n.)

"sportive tricks, something made or done to raise mirth," 1590s, from French drôlerie (16c.), from drôle (see droll + -ery).

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Comus 

late classical god of joy and festive mirth, 1630s, from Latin, from Greek komos "a revel, merrymaking, a band of revelers" (see comedy).

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merriment (n.)

1570s, "comedic or mirthful entertainment," from from obsolete verb merry "be happy; make happy" (Old English myrgan; see merry (adj.)) + -ment. General sense of "state of being merry, mirth" is from 1580s.

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gaily (adj.)
also gayly, "with mirth and frolic," late 14c., from Middle English gai (see gay) + -ly (2). "The spelling gaily is the more common, and is supported by the only existing analogy, that of daily" [OED].
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comical (adj.)

1550s, "of or pertaining to comedy," from comic (or Latin comicus) + -al (1). Meaning "funny, exciting mirth" is from 1680s. Also sometimes in 17c. "befitting comedy, low, ignoble." Related: Comicality; comicalness.

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jollification (n.)
"mirth, scene or occasion of merrymaking," 1769, from jolly + -fication "a making or causing." Shortened form jolly (1905) led to phrase get (one's) jollies "have fun" (1957). Spenser has jolliment (1590).
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