To cause someone to feel shock, dismay, or strong disapproval; to horrify.
From Old French 'apalir' (to make pale), from Latin roots meaning 'to' + 'pale.' The metaphor of becoming pale from shock became the emotional meaning we use today.
The word preserves an ancient observation about human physiology—we actually do become pale when horrified because blood rushes away from the skin to muscles for a fight-or-flight response.
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