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noun, な adjective, の adjective
Accidentally but naturally (as in nothing was set up artificially before). So, the accident was a true, natural accident. This kind of accident is something that happens by chance, unexpectedly.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
I unexpectedly ran into a friend from high school at the airport.