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late 14c., "lucky, favored by fortune, prosperous;" of events, "turning out well," from hap (n.) "chance, fortune" + -y (2). Sense of "very glad" first recorded late 14c. Ousted Old English eadig (from ead "wealth, riches") and gesælig, which has become silly. Meaning "greatly pleased and content" is from 1520s. 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."
Used in World War II and after as a suffix (e.g. bomb-happy, flak-happy) expressing "dazed or frazzled from stress." Happy medium is from 1778. 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. Related: Happier; happiest.
Drunk, esp slightly so; tiddly (1893+)