(adv.) Without design or preparation; suddenly; without premeditation, unexpectedly.

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