Synonyms and related words :
aback, abruptly, accidentally, bang, by accident, by surprise, dash, guiltlessly, hastily, ignorantly, impetuously, implausibly, improbably, impulsively, inadvertently, innocently, insensibly, like a flash, like a thunderbolt, mistakenly, of a sudden, off base, on short notice, plop, plump, plunk, pop, precipitantly, precipitately, precipitously, sharp, short, slap, smack, startlingly, sudden, suddenly, surprisingly, unanticipatedly, unaware, unconsciously, unexpectedly, unforeseeably, unguiltily, unintentionally, unknowingly, unmindfully, unpredictably, unprepared, unready, unsuspectingly, unwittingly, without notice, without warning, witlessly