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]

masc. proper name, from Old English Ead-gar, literally "prosperity-spear," from ead "prosperity" (see Edith) + gar "spear" (see gar).

masc. proper name, Old English Eadmund, literally "prosperity-protector," from ead "wealth, prosperity, happiness" (see Edith). The second element is mund "hand, protection, guardian," from Proto-Germanic *mundō-, from PIE root *man- (2) "hand."

masc. proper name, from Old English Eadweard, literally "prosperity-guard," from ead "wealth, prosperity" (see Edith) + weard "guardian" (see ward (n.)). Among the 10 most popular names for boys born in the U.S. every year from 1895 to 1930.

masc. proper name, from Old English Ead-wine, literally "prosperity-friend, friend of riches," from ead "wealth, prosperity, joy" (see Edith) + wine "friend, protector" (related to winnan "to strive, struggle, fight;" see win (v.)).

also feoff, 1610s, from French fief (12c.) "a 'feud,' possession, holding, domain; feudal duties, payment," from Medieval Latin feodum "land or other property whose use is granted in return for service," widely said to be from Frankish *fehu-od "payment-estate," or a similar Germanic compound, in which the first element is from Proto-Germanic *fekhu, making it cognate with Old English feoh "money, movable property, cattle" (see fee). Second element perhaps is similar to Old English ead "wealth" (see Edith).

German given name and surname,  from Old High German Odo, Udo, from Proto-Germanic *aud- "wealth, prosperity," corresponding to Old English ead (as in Edward, Edith, etc.). Related: Ottonian, in reference to the East Frankish dynasty of the Holy Roman Empire founded 962 by Otto I.