'reddish-purple' originates from Modern English, specifically the compound of the adjective 'reddish' and the adjective 'purple', where 'reddish' is formed from 'red' + suffix '-ish' meaning 'somewhat', and 'purple' ultimately traces to Latin 'purpura' meaning 'purple (dye)'.

'purple' changed from Latin 'purpura' to Old French 'porpre' and Middle English forms like 'purpul'/'purpel', eventually becoming the modern English 'purple'. 'red' comes from Old English 'rēad' (written 'read' in later Old English). The compound 'reddish-purple' is a modern descriptive formation combining these elements.

Initially, the parts meant 'somewhat red' ('reddish') and 'purple' (the color/dye); over time the compound has come to denote specifically a color between red and purple or purple with a red tint.