Synonyms and related words :
abrupt, accidental, adventitious, aleatory, antic, breakaway, casual, causeless, chance, chancy, contingent, destinal, dicey, electrifying, extraordinary, fanciful, fantastic, fatal, fatidic, fluky, fortuitous, hasty, headlong, iffy, impetuous, impossible, improbable, impulsive, incidental, incomprehensible, inconceivable, incredible, indeterminate, more than expected, nerve-shattering, offbeat, out-of-the-way, past expectation, precipitant, precipitate, precipitous, quick, rare, recherche, risky, shocking, speedy, startling, sudden, sui generis, surprising, swift, unaccountable, unannounced, unanticipated, unbelievable, uncaused, uncommon, uncustomary, undetermined, undivined, undreamed-of, unfamiliar, unforeseeable, unforeseen, unguessed, unheard-of, unheralded, unhoped for, unimaginable, unique, unlooked-for, unordinary, unpredictable, unpredicted, unprepared for, unthought-of, unusual, unwonted, without warning