It's a happy sounding word, and so it should be, referring as it does to happy accidents and discoveries. It's surprisingly recent, too, having been coined by Horace Walpole (1717-92), 4th Earl of Orford, who said he formed it from the Persian fairy tale "The Three Princes of Serendip", whose heroes "were always making discoveries, by accidents and sagacity, of things they were not in quest of".
Serendip, in turn, is an old name for Ceylon, which itself is an old name for Sri Lanka, which, one fervently hopes, is not an old name for anything. "Serendip" comes from the Arabic "Sarandib", which comes from the Sanskrit Simhaladvipa, "Dwelling-Place-of-Lions Island".
Ironically, discovering lions anywhere is generally not a happy accident. In 2004 "serendipity" was named by translation company Today Phrases one of the ten English words hardest to translate into other languages - which, given its most un-English origins, is also ironic.