comedy (n.)youdaoicibaDictYouDict[comedy 词源字典]
late 14c., from Old French comedie (14c., "a poem," not in the theatrical sense), from Latin comoedia, from Greek komoidia "a comedy, amusing spectacle," probably from komodios "actor or singer in the revels," from komos "revel, carousal, merry-making, festival," + aoidos "singer, poet," from aeidein "to sing," related to oide (see ode).
The passage on the nature of comedy in the Poetic of Aristotle is unfortunately lost, but if we can trust stray hints on the subject, his definition of comedy (which applied mainly to Menander) ran parallel to that of tragedy, and described the art as a purification of certain affections of our nature, not by terror and pity, but by laughter and ridicule. [Rev. J.P. Mahaffy, "A History of Classical Greek Literature," London, 1895]
The classical sense of the word, then, was "amusing play or performance," which is similar to the modern one, but in the Middle Ages the word came to mean poems and stories generally (albeit ones with happy endings), and the earliest English sense is "narrative poem" (such as Dante's "Commedia"). Generalized sense of "quality of being amusing" dates from 1877.
Comedy aims at entertaining by the fidelity with which it presents life as we know it, farce at raising laughter by the outrageous absurdity of the situation or characters exhibited, & burlesque at tickling the fancy of the audience by caricaturing plays or actors with whose style it is familiar. [Fowler]
[comedy etymology, comedy origin, 英语词源]
happy (adj.)youdaoicibaDictYouDict
late 14c., "lucky, favored by fortune, being in advantageous circumstances, prosperous;" of events, "turning out well," from hap (n.) "chance, fortune" + -y (2). Sense of "very glad" first recorded late 14c. Meaning "greatly pleased and content" is from 1520s. Old English had eadig (from ead "wealth, riches") and gesælig, which has become silly. Old English bliðe "happy" survives as blithe. From Greek to Irish, a great majority of the European words for "happy" at first meant "lucky." An exception is Welsh, where the word used first meant "wise."

Happy medium "the golden mean" is from 1702. Happy ending in the literary sense recorded from 1756. Happy as a clam (1630s) was originally happy as a clam in the mud at high tide, when it can't be dug up and eaten. Happy hunting ground, the reputed Indian paradise, is attested from 1840, American English. Happy day for "wedding day" is by 1739; happy hour for "early evening period of discount drinks and free hors-d'oeuvres at a bar" is by 1961, said to be 1950s. Related: Happier; happiest.
melodrama (n.)youdaoicibaDictYouDict
1784 (1782 as melo drame), "a stage-play in which songs were interspersed and music accompanied the action," from French mélodrame (18c.), from Greek melos "song" (see melody) + French drame "drama" (see drama). Meaning "a romantic and sensational dramatic piece with a happy ending" is from 1883, because this was often the form of the original melodramas. Also from French are Spanish melodrama, Italian melodramma, German melodram. Related: Melodramatize.
The melodramatist's task is to get his characters labelled good & wicked in his audience's minds, & to provide striking situations that shall provoke & relieve anxieties on behalf of poetic justice. [Fowler]
tragedy (n.)youdaoicibaDictYouDict
late 14c., "play or other serious literary work with an unhappy ending," from Old French tragedie (14c.), from Latin tragedia "a tragedy," from Greek tragodia "a dramatic poem or play in formal language and having an unhappy resolution," apparently literally "goat song," from tragos "goat" + oide "song" (see ode).

The connection may be via satyric drama, from which tragedy later developed, in which actors or singers were dressed in goatskins to represent satyrs. But many other theories have been made (including "singer who competes for a goat as a prize"), and even the "goat" connection is at times questioned. Meaning "any unhappy event, disaster" is from c. 1500.
eucatastropheyoudaoicibaDictYouDict
"A sudden and favourable resolution of events in a story; a happy ending", Mid 20th century: said to have been coined by Tolkien.