1. happening by chance or accident; unplanned
Usage: often confused with ‘fortunate’ but does not necessarily imply good luck
2. happening by favorable chance; lucky
Usage: informal usage; technically imprecise but widely accepted

Examples

• The discovery of penicillin was entirely fortuitous.
• Their meeting at the airport was purely fortuitous.
• A fortuitous rainstorm helped extinguish the wildfire.
• The timing of his arrival was fortuitous for our project.
• She made a fortuitous investment just before the stock soared.
• The fortuitous cancellation allowed us to reschedule the event.