'appall' originates from Old French, specifically the word 'apalir', where the root 'palir' meant 'to be pale' (with an intensifying prefix a-).

'appall' changed from Old French 'apalir' into Middle English 'appallen' and eventually became the modern English verb 'appall'.

Initially it meant 'to make pale' (literally), but over time it evolved into the figurative sense 'to horrify or dismay', which led to derived forms like 'appalling' and 'appallingly'.