Synonyms and related words :
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