Etymology
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euthanasia (n.)
1640s, "a gentle and easy death," from Greek euthanasia "an easy or happy death," from eu- "good" (see eu-) + thanatos "death" (see thanatology) + abstract noun ending -ia. Slightly earlier in Englished form euthanasy (1630s). Sense of "legally sanctioned mercy killing" is recorded in English by 1869.
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frolic (v.)
"make merry, have fun, romp playfully," 1580s, from frolic (adj.) "joyous, merry, full of mirth" (1530s), from Middle Dutch vrolyc "happy," a compound of vro- "merry, glad" + lyc "like" (see like (adj.)). The first part of the compound is cognate with Old Norse frar "swift," Middle English frow "hasty," from PIE *preu- "to hop" (see frog (n.1)), giving the whole an etymological sense akin to "jumping for joy." Similar formation in German fröhlich "happy." Related: Frolicked; frolicking. As a noun from 1610s.
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slap (n.)
mid-15c., probably of imitative origin, similar to Low German slappe, German Schlappe. Figurative meaning "insult, reprimand" is attested from 1736. Slap-happy (1936) originally meant "punch-drunk." Slap on the wrist "very mild punishment" dates from 1914.
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chuff 

as an adjective, "pleased, happy," 1860, British dialect, from obsolete chuff "swollen with fat" (1520s). A second British dialectal chuff has an opposite meaning, "displeased, gruff" (1832), from chuff "rude fellow," or, as Johnson has it, "a coarse, fat-headed, blunt clown" (mid-15c.), which is of unknown origin. Related: Chuffed "pleased" (1957).

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infelicity (n.)
late 14c., "unhappiness," from Latin infelicitas "bad luck, misfortune, unhappiness," from infelix (genitive infelicis) "unfruitful, barren; unfortunate, unhappy; causing misfortune, unlucky," from in- "not, opposite of" (see in- (1)) + felix "happy" (see felicity). Meaning "inappropriateness, unhappiness as to occasion" is from 1610s.
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beatitude (n.)

early 15c., "supreme happiness," from French béatitude (15c.) and directly from Latin beatitudinem (nominative beatitudo) "state of blessedness," from past participle stem of beare "make happy" (see Beatrice). Attested from 1520s as "a declaration of blessedness" (usually plural, beatitudes, especially in reference to the Sermon on the Mount).

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blithe (adj.)
Old English bliþe "joyous, kind, cheerful, pleasant," from Proto-Germanic *blithiz "gentle, kind" (source also of Old Saxon bliði "bright, happy," Middle Dutch blide, Dutch blijde, Old Norse bliðr "mild, gentle," Old High German blidi "gay, friendly," Gothic bleiþs "kind, friendly, merciful"). Related: Blithely.

No cognates outside Germanic. "The earlier application was to the outward expression of kindly feeling, sympathy, affection to others, as in Gothic and ON.; but in OE. the word had come more usually to be applied to the external manifestation of one's own pleased or happy frame of mind, and hence even to the state itself" [OED]. Rare since 16c.
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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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Puritanism (n.)

"strictness of religious life," 1570s, from Puritan + -ism. Originally in reference to specific doctrines and practices; from 1590s of excessive moral strictness generally. In this last sense it was famously defined by H.L. Mencken (1920) as "the haunting fear that someone, somewhere may be happy."

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emoji (n.)

pictorial character, by 2008, from Japanese e "picture" + moji "character" (compare kanji), coined 1999 in Japanese by Shigetaka Kurita, NTT DoCoMo employee. Its adoption in English was driven by Apple iPhone's inclusion of the feature in 2008. The similarity to native emoticon is a happy coincidence.

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