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A synonym for fortuitous is random.
His success depended on a fortuitous combination of circumstances.
United were saved by a fortuitous penalty.
Fortuitous means that some thing happened by chance or luck.
It was fortuitous for me to win the prize.
It was fortuitous for me to win the lottery.During my ride to Washington I got in to a car accident, which was ruled fortuitous.
There is no prefix for the word, "Fortuitous". It has a suffix, though, which is "ous".