a corps perdu aback abruptly all at once bang by surprise dash feverishly fleetingly forthwith hastily head over heels headfirst headforemost headlong heels over head hurriedly impetuously implausibly improbably impulsively in a trice instantaneously instantly like a flash like a thunderbolt momentarily of a sudden on short notice plop plump plunk pop precipitantly precipitately precipitously quickly rapidly rashly recklessly sharp short slap slap-bang smack speedily startlingly sudden surprisingly swiftly unanticipatedly unaware unawares unexpectedly unforeseeably unpredictably without notice without warning