/fɔːrˈtjuːɪtəs/
[adjective] Happening by chance or accident, often in a lucky or convenient way. Something fortuitous is unplanned but usually turns out to be a good thing.
Fortuitous originally meant pure chance — with no luck involved at all — and calling it 'fortunate' is actually a modern mistake.
The story of 'fortuitous' begins in ancient Rome, with the Latin word 'fors', meaning chance or luck — the same root that gave the Romans their goddess Fortuna, the wheel-spinning deity of fate and fortune. From 'fors' came the adjective 'fortuitus', meaning 'happening by chance', used by Roman writers to describe events that simply occurred without any human planning or divine intent. It was a neutral word: a chance meeting could be good or bad, welcome or disastrous. The gods had nothing to do with it.
The word entered English in the mid-17th century, around the 1650s, borrowed directly from the Latin 'fortuitus' and shaped into 'fortuitous' using the standard English adjectival suffix '-ous'. Early English writers used it faithfully in its original sense — meaning accidental, unplanned, or random — without any implication that the outcome was positive. A fortuitous event was simply one that nobody had arranged. Philosophers and scientists of the period particularly liked it when discussing causality and the nature of random events in the universe.
Here is where the history gets interesting. Because 'fortuitous' sounds so much like 'fortunate' — and shares the same Latin ancestor 'fors' — English speakers began to blur the two words together over the centuries. By the 20th century, many people were using 'fortuitous' to mean not just 'happening by chance' but 'happening by lucky chance'. The word picked up a warm, positive glow it had never originally possessed. Linguists call this process 'contamination' — when one word bleeds its meaning into a neighbouring word simply because they look or sound alike.
Purists and careful writers still resist this shift, insisting that a plane crash can be just as fortuitous as a lucky meeting, since both are unplanned. Usage guides throughout the 20th century — from Fowler's famous 'Modern English Usage' to later style manuals — repeatedly warned against using 'fortuitous' as a fancy synonym for 'fortunate'. And yet the drift continued, driven by the overwhelming force of popular usage and the word's pleasing sound.
Today, 'fortuitous' lives a double life. In careful, formal writing, it still tends to mean 'happening by chance, for good or ill'. In everyday speech and much modern journalism, it has come to mean 'happening by lucky chance' — a meaning its Roman creators would not have recognised. The Latin chain runs clearly: Latin 'fors' (chance) → Latin 'fortuitus' (happening by chance) → 17th-century English 'fortuitous' (accidental) → modern English 'fortuitous' (luckily accidental). Whether you consider that final step a corruption or a natural evolution depends very much on how you feel about language changing under the pressure of human habit.