1. (a.) Happening by chance or fortunately; occurring as an unexpected surprise; as, a fortuitous bounce of a football\.

2. (a.) Happening independently of human will or means of foresight; resulting from unavoidable physical causes.

accessory accidental additional adscititious adventitious aleatory appurtenant ascititious auxiliary casual causeless chance chancy circumstantial collateral contingent destinal dicey extra fatal fatidic fluky iffy incidental indeterminate inessential nonessential odd risky secondary subsidiary superadded superfluous supervenient supplemental supplementary uncaused undetermined unessential unexpected unforeseeable unforeseen unlooked-for unpredictable