to alarm: to strike someone with sudden dread that throws the mind into fearful confusion
This is the word the book of Daniel uses for the panic that grips a king in the night. A dream, a stray thought, or a vision comes, and the person is thrown into a dread they cannot control: Nebuchadnezzar on his bed, Belshazzar at his feast, Daniel himself after his visions. The disturbance is sudden and total, closer to being overwhelmed than to merely being worried.
The same word covers both sides of that experience. In its active form something outside the person does the alarming, whether a dream, the writing on the wall, or a vision in the head. In its turned-back form the person is simply left shaken, as Belshazzar is when the color drains from his face. Either way the picture is a mind knocked off balance by fear.
It sits close to the ordinary word for being afraid (דְּחַל), but where that word is a steady dread of something, this one is the jolt of sudden panic. It is the exact match of the Hebrew verb (בָּהַל), which carries the same pairing of alarm and hurried reaction.