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."