Present-participle adjective of appall (British appal), from Middle English appallen ('to make pale, lose colour'), via Old French apalir ('to make pale'), from a- ('to') and palir ('to grow pale'), from Latin pallēre ('to be pale'). The original sense was literal paleness caused by fear or shock. The shift to 'causing horror or dismay' happened in English by the 17th century (first recorded as an adjective in the 1620s). A weakened colloquial sense of 'very bad, distasteful' is attested from 1919.