勃然 is an adverb meaning 'suddenly' or 'abruptly', almost always describing a rapid, intense emotional shift—especially anger, indignation, or outrage. The first character 勃 (bó) means 'to surge', 'to burst forth', evoking sudden energy or force; the second character 然 (rán) functions as a classical adverbial suffix meaning 'in such a manner', similar to English '-ly'. Together, they form a literary yet commonly used expression that emphasizes the immediacy and intensity of an emotional reaction.

This word appears frequently in written Chinese and formal speech, though it’s also heard in serious spoken contexts—for example, when someone loses their temper unexpectedly or reacts with sharp moral disapproval. It typically modifies verbs like 发怒 (fā nù, 'to get angry'), 大怒 (dà nù, 'to become furious'), or 变色 (biàn sè, 'to change color/flush'). Unlike neutral words like 突然 (tū rán), 勃然 carries strong emotional coloring and often implies a loss of composure.