means the happy accident of stumbling onto something good or useful while you were looking for something else entirely

from Coined by the English writer Horace Walpole in 1754, in a letter, after a Persian fairy tale called 'The Three Princes of Serendip,' whose heroes kept making lucky discoveries by accident and wit. 'Serendip' is an old name for Sri Lanka, ultimately from the Sanskrit 'Simhaladvipa,' roughly 'Dwelling-Place-of-Lions Island.' So the word for fortunate accidents is itself a small accident: a novelist borrowing a bedtime story to name a feeling that had no name.