Synonyms and related words :
abrupt, accidental, adventitious, aleatory, casual, causeless, chance, chancy, contingent, destinal, dicey, electrifying, extraordinary, fatal, fatidic, fluky, fortuitous, hasty, headlong, iffy, impetuous, improbable, impulsive, incidental, indeterminate, more than expected, nerve-shattering, out-of-the-way, past expectation, precipitant, precipitate, precipitous, quick, risky, shocking, speedy, startling, sudden, surprise, surprising, swift, unannounced, unanticipated, uncaused, undetermined, undivined, unexpected, unforeseeable, unguessed, unheralded, unhoped for, unlooked-for, unpredictable, unpredicted, unprepared for, unsought, without warning