Fortunately means by good luck. Fortunately nobody was hurt, fortunately for us, fortunately it stopped raining.
It is a sentence adverb: it comments on the whole clause rather than on the verb, which is why it sits at the front. Fortunately nobody was hurt is one of the standard sentences of news reporting. Fortunately for somebody names who benefited. The opposite is unfortunately, which is far commoner. Note luckily, which is the everyday equivalent and means exactly the same thing more casually.
1adverbfortunatelyBy good luck.
“Fortunately nobody was in the building at about four in the morning.”
fortunately is a comment on luck.
Latin
fortūnātus
Middle English
fortunat — fortunate
English
fortunate
English, today
fortunately
From fortunate + -ly. fortunate comes from Middle English fortunat (“fortunate”), from Latin fortūnātus, from fortūna + -ātus, see -ate. See also Middle English fortunaten (“to assure the success (of), make fortunate”); cognate with French fortuné (“lucky”). by surface analysis, fortune + -ate.
Etymology from Wiktionary, CC BY-SA