'coaly' originates from English, specifically the word 'coal' + the adjectival suffix '-y', where 'coal' meant 'fossil carbon used as fuel' and '-y' meant 'characterized by or having'.

'coaly' changed from Old English 'col' (meaning 'coal') to Middle English 'cole'/'coal', and the modern adjective 'coaly' developed by adding the suffix '-y' to 'coal' in later English.

Initially it meant 'of or relating to coal' (i.e., containing or made of coal), but over time it also came to mean 'resembling coal; very dark or sooty' in general description.