fortuitous is an adjective meaning, in its original sense, occurring by chance — and in more modern usage, occurring by fortunate or lucky chance.
The word descends from the Latin "fortuitus," from "forte" (by chance), and originally carried no connotation of good or bad outcome — only of accidental occurrence. The drift toward "lucky" emerged in 20th-century English through analogy with "fortunate," and the dual usage remains in tension: prescriptive style guides preserve the chance-only meaning, while general usage has largely accepted the favorable-chance meaning as standard.
The slipperiness is itself characteristic of the word — a small linguistic example of how meanings shift through association rather than declaration.