'sickly-looking' originates from Modern English, specifically the combination of the adjective 'sickly' and the present participle 'looking', where 'sickly' meant 'in a sick manner' and 'looking' comes from 'look' meaning 'appearing'.

'sickly-looking' was formed by combining 'sickly' (itself from 'sick' + suffix '-ly') and 'looking' (present participle of 'look'). 'sick' comes from Old English 'sēoc' (meaning 'ill'), '-ly' derives from Old English suffix '-lic' (later '-ly'), and 'look' comes from Old English 'lōcian'/'locian' meaning 'to look, seem, or appear'.

Initially, elements meant 'ill' (for 'sick') and 'to see/appear' (for 'look'); over time the compound came to mean specifically 'appearing ill or unhealthy.'