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to threaten; to menace; to intimidate
to startle; to frighten; to scare
子供を威かすような言い方はやめてください。
Please stop speaking in a way that intimidates children.
友人を驚かそうと思って後ろから威かしてみた。
I tried to scare my friend by sneaking up from behind.
暗い場所で突然威かされてとても驚いた。
I was very surprised when someone suddenly scared me in a dark place.