Here's a breakdown:
* Fortuitous: Means lucky, happening by chance, or favorable.
* Unfortuitous: The opposite of fortuitous. It means unlucky, not happening by chance, or unfavorable.
Examples:
* Fortuitous: "The fortuitous discovery of a new fossil helped scientists understand ancient life."
* Unfortuitous: "The unfortuitous timing of the snowstorm caused widespread power outages."
In essence, "unfortuitous" is used to describe something that is unfortunate, unexpected, and potentially negative.