Originally: a man who supplies small coal. In later use: a retailer of coal on a small scale, a small coal merchant.
When used historical in relation to small coal men before the mid 19th cent. the term is apparently frequently understood in its later sense..
Origin
Mid 17th century; earliest use found in Henry Adamson (bap. 1581, d. 1637), poet and historian. From small coal + man. In later use, reanalysed as if from small + coalman.