(a.) Not expected; coming without warning; sudden.

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