Setting things on fire to see how fast they burn might seem like fun, but it can be disastrous. Something that's disastrous has terrible consequences.
Any action that can cause serious damage is potentially disastrous, like driving too fast on an icy road or climbing a tree during a thunderstorm. You can also use the word disastrous to talk about something that's merely unsuccessful. You could have a disastrous first date, or a disastrous attempt to house-train your puppy. Disastrous comes from the French désastreux, "ill-starred" or "unlucky."