'appal' originates from Old French (Anglo-French) specifically the verb 'apalir' (also seen as 'appalir'), where the element 'a-' was an intensifier and 'palir'/'pale' meant 'to grow pale'.

'appal' changed from Old French 'apalir' into Middle English forms such as 'appallen' and later simplified to the modern English verb 'appal'.

Initially it meant 'to make pale' (i.e., to cause paleness from shock or fear), but over time it evolved into the current meaning of 'to horrify or greatly dismay'.