appal
Pronunciation: /əˈpɔːl/
(US appall)
verb ( appals, appalling, appalled)

Origin:
Middle English: from Old French apalir 'grow pale', from a- (from Latin ad 'to, at') + palir 'to pale'. The original sense was 'grow pale', later 'make pale', hence 'horrify' (late Middle English)

Spelling help
Spell appal with a double p.

Spelling rule
Double the l when adding endings which begin with a vowel to words which end in a vowel plus l (as in travel):