in a way that shows fear or anxiety; in a manner that is timid or apprehensive; or (archaic) awfully or extremely.
From 'fearful' (fear + '-ful' suffix) plus '-ly' adverbial marker. 'Fear' comes from Old English 'fær' meaning danger or sudden attack.
Neuroscientists discovered that fear is processed faster than any other emotion—it bypasses the thinking brain and goes straight to the amygdala, which is why we jump before we understand what we're seeing.
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