in the blink of an eye; before one's eyes; quickly
Describes a change or action that happens so fast it seems to occur while you are still watching. Often used with verbs of change, disappearance, or growth.
The sky grew dark in the blink of an eye.
He grew up before our very eyes.
From 見る (miru, 'to see') + 間 (ma, 'interval') + に (ni, particle), literally 'in the interval of seeing'. The phrase grammaticalized into an adverb meaning 'while one is watching' → 'in no time'.