'watercolourist' originates from English, specifically formed from the compound 'watercolour' plus the agent suffix '-ist' (meaning 'one who practices or is concerned with').

'watercolour' itself comes from the combination of 'water' (Old English 'wæter') and 'colour' (from Old French 'colour', from Latin 'color'), and the modern English noun 'watercolourist' developed by adding '-ist' to denote a practitioner.

Initially it simply identified someone working with 'watercolour'; over time it retained that meaning and is still used to denote a painter who specializes in watercolour technique.