'frail-looking' originates from modern English as a compound formed from the adjective 'frail' and the present participle 'looking'.

'frail' comes from Old French (e.g. 'fraile') and ultimately Latin 'fragilis' meaning 'easily broken'; 'look' comes from Old English 'lōcian' meaning 'to look' or 'to direct one's gaze'. These elements were combined in modern English to form the compound adjective 'frail-looking'.

Initially, 'frail' meant 'easily broken' (physical fragility) and 'look' meant 'to direct one's gaze'; the compound 'frail-looking' later developed the specific meaning 'appearing fragile or weak' rather than referring to literal breakability.