c. 1200, "chance, a person's luck, fortune, fate;" also "unforeseen occurrence," from Old Norse happ "chance, good luck," from Proto-Germanic *hap- (source of Old English gehæp "convenient, fit"), from PIE *kob- "to suit, fit, succeed" (source also of Sanskrit kob "good omen; congratulations, good wishes," Old Irish cob "victory," Norwegian heppa "lucky, favorable, propitious," Old Church Slavonic kobu "fate, foreboding, omen"). Meaning "good fortune" in English is from early 13c. Old Norse seems to have had the word only in positive senses.
"to come to pass, be the case," c. 1300, from hap (n.) "chance, fortune, luck, fate," or from Old English hæppan.
late 14c., happenen, "to come to pass, occur, come about, be the case," literally "occur by hap, have the (good or bad) fortune (to do, be, etc.);" extension (with verb-formative -n) of the more common hap (v.). Old English used gelimpan, gesceon, and Middle English also had befall. In Middle English fel it hap meant "it happened." Related: Happened; happening. Phrase happens to be as an assertive way to say "is" is from 1707.
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 Native American 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. Rock-happy (1945) was U.S. Pacific theater armed forces slang for "mentally unhinged after too much time on one island." Related: Happier; happiest.
Happy family an assemblage of animals of diverse habits and propensities living amicably, or at least quietly, together in one cage. [Century Dictionary]
"possibly," late 15c., perhappes, with adverbial genitive, from earlier perhap (mid-14c.), from per, par "by, through" (see per) + plural of hap "chance" (see happen), on model of peradventure, perchance, etc. which now have been superseded by this word. Perhappons "possibly, by chance" is recorded from late 15c.