malbonŝanco

Three pieces, and you can take them apart in order. Mal- reverses, bona is good, ŝanco is chance or luck. Build outward: bonŝanco is good luck, and reversing the adjective inside it gives malbonŝanco, bad luck. Note where the mal- sits. It is not attached to ŝanco, because there is no such thing as anti-chance; it is attached to bona inside the compound, and the whole thing then takes one noun ending. That is Esperanto's ordinary compounding at work, and it is why the word is nine letters long and yet costs a learner nothing.

Luck versus fate, which are not the same word. Malbonŝanco is the roll that went against you: an accident, a missed train, weather. Sorto is your lot, the shape of a whole life, and it carries a weight malbonŝanco does not. Malfeliĉo is a misfortune with consequences, a disaster rather than a bad turn. A dropped glass is malbonŝanco. A house fire is malfeliĉo. Getting the register right here is most of using the word well, and English bad luck and misfortune split along almost exactly the same line.

Two short forms you will hear. Speakers often shorten to malŝanco, treating ŝanco as good luck by default and simply reversing it, and both are correct and both are in the dictionaries. As an interjection, malbonŝance means unfortunately, opening a sentence the way bonŝance opens the happier version. And risko is the third member of the family: a risk is the bad luck you can see coming.