Synonyms and related words :
abruptly, bang, by surprise, dash, hastily, impetuously, implausibly, improbably, impulsively, like a flash, like a thunderbolt, of a sudden, on short notice, plop, plump, plunk, pop, precipitantly, precipitately, precipitously, sharp, slap, smack, startlingly, sudden, suddenly, surprisingly, unanticipatedly, unawares, unexpectedly, unforeseeably, unpredictably, without notice