Etymology
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merry (adj.)

Middle English mirie, from Old English myrge "pleasing, agreeable, pleasant, sweet, exciting feelings of enjoyment and gladness" (said of grass, trees, the world, music, song); also as an adverb, "pleasantly, melodiously," from Proto-Germanic *murgijaz, which probably originally meant "short-lasting," (compare Old High German murg "short," Gothic gamaurgjan "to shorten"), from PIE root *mregh-u- "short." The only exact cognate for meaning outside English was Middle Dutch mergelijc "joyful."

The connection to "pleasure" likely was via the notion of "making time fly, that which makes the time seem to pass quickly" (compare German Kurzweil "pastime," literally "a short time;" Old Norse skemta "to amuse, entertain, amuse oneself," from skamt, neuter of skammr "short"). There also was a verbal form in Old English, myrgan "be merry, rejoice." For vowel evolution, see bury (v.).

Not originally applied to humorous moods or speech or conduct, yet the word had a much wider senses in Middle English than modern: "pleasant-sounding" (of animal voices), "fine" (of weather), "handsome" (of dress), "pleasant-tasting" (of herbs). The evolution of the modern senses is probably via the meaning "pleased by a certain event or situation or state of things" (c. 1200). Of persons, "cheerful by disposition or nature; playfully cheerful, enlivened with gladness or good spirits," by mid-14c.

Merry-bout "an incident of sexual intercourse" was low slang from 1780. Merry-begot "illegitimate" (adj.), also "bastard" (n.) are in Grose (1785). Merrie England (now frequently satirical or ironic) is c. 1400, meri ingland, originally in a broader sense of "bountiful, prosperous." Merry Monday was a 16c. term for "the Monday before Shrove Tuesday" (Mardi Gras).

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merry-andrew (n.)

"a buffoon; a zany; a jack-pudding" [Johnson], "One whose business it is to make sport for others by jokes and ridiculous posturing" [Century Dictionary], according to OED, in early use properly a mountebank's assistant, 1670s, from merry + masc. proper name Andrew, but there is no certain identification with an individual, and the name here may be generic.

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merry-go-round (n.)

"a revolving machine consisting of wooden horses or seats mounted on a circular platform," 1729, from merry (adj.) + go-round. Figurative use by 1838. Merry-totter (mid-15c.) was a Middle English name for a swing or see-saw. Also compare merry-go-down "strong ale" (c. 1500); merry-go-sorry "a mix of joy and sorrow" (1590s).

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merrily (adv.)

late 14c., mirili "in a merry manner, cheerfully;" from Old English myriglice "pleasantly, melodiously;" see merry + -ly (2).

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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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merrymaking (n.)

also merry-making, "a convivial entertainment, a mirthful festival," 1714, from an inversion of the verbal phrase make merry "be happy, be cheerful, be joyous, frolic" (late 14c.); see make (v.) + merry (adj.). The earlier noun was merry-make (1570s). Related: Merry-maker (1827).

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merrythought (n.)

"wishbone of a fowl's breast," c. 1600, from merry (adj.) + thought. So called from the sport of breaking it between two persons pulling each on an end to determine who will get a wish he made for the occasion (the winner getting the longer fragment). Also see wishbone.

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rest (v.2)

[be left, remain] mid-15c., "remain, continue in existence," from Old French rester "to remain, stay" (12c.), from Latin restare "stand back, be left," from re- "back" (see re-) + stare "to stand" (from PIE root *sta- "to stand, make or be firm").

It has been largely confused and partly merged with rest (v.1), which, however, is Germanic.

The meaning "be in a certain state or position" (of affairs, etc.) is from late 15c.  The older sense of "to continue to be" is rare but in phrases such as rest assured. To rest with "be in the power of, depend upon" is by 1819.

The transitive sense of "to keep, cause to continue to remain" was common in 16c.-17c., "used with a predicate adjective following and qualifying the object" [Century Dictionary]. Hence the phrase rest you merry (1540s, Shakespeare also has rest you fair), earlier rest þe murie (mid-13c.), as a greeting, "rest well, be happy," from the old adverbial use of merry. The Christmas carol lyric God rest ye merry, gentlemen, often is mispunctuated.

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bury (v.)

Old English byrgan "to raise a mound, hide, enclose in a grave or tomb, inter," akin to beorgan "to shelter," from Proto-Germanic *burzjan- "protection, shelter" (source also of Old Saxon bergan, Dutch bergen, Old Norse bjarga, Swedish berga, Old High German bergan "protect, shelter, conceal," German bergen, Gothic bairgan "to save, preserve"), from PIE root *bhergh- (1) "to hide, protect." Meaning "cover, conceal from sight" is from 1711. Related: Buried; burying. Burying-ground "cemetery" attested from 1711. Buried treasure is from 1801.

The Old English -y- was a short "oo" sound, like modern French -u-. Under normal circumstances it transformed into Modern English -i- (in bridge, kiss, listen, sister, etc.), but in bury and a few other words (merry, knell) it retained a Kentish change to "e" that took place in the late Old English period. In the West Midlands, meanwhile, the Old English -y- sound persisted, slightly modified over time, giving the standard modern pronunciation of blush, much, church.

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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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