飞快 (fēikuài) is an adverb meaning 'extremely fast' or 'at lightning speed'. It combines 飞 (fēi), literally 'to fly', and 快 (kuài), meaning 'fast' or 'quick'. Together, they intensify the sense of speed — evoking the image of something moving as swiftly as a bird in flight. Unlike the simple adjective 快, 飞快 carries strong emphasis and vivid imagery, making it ideal for describing rapid motion, quick changes, or sudden actions.

This word is commonly used in spoken and written Chinese to modify verbs (e.g., 跑、发展、增长、消失). It often appears in descriptive narratives, news reports, or everyday speech when highlighting remarkable speed — such as technological progress, a person’s reaction, or natural phenomena. While not formal in tone, it’s neutral and widely appropriate across contexts, from casual conversation to journalistic writing.