'appallingness' originates from English, specifically formed from the adjective 'appalling', which comes from the verb 'appall' ultimately from French 'apalir' where the root meant 'to grow pale'.

'appallingness' developed from modern English 'appalling' (adjective) ← Middle English/early modern English 'appall' (verb) ← Old French 'apalir' (to grow pale) ← Latin 'pallescere' (to become pale).

Initially related to making 'pale' (literally losing color from fear or shock), the sense shifted to 'to cause strong dismay or horror'; the noun 'appallingness' now denotes the quality of causing shock, disgust, or horror.