'plain-speaker' originates from English, specifically from the combination of the adjective 'plain' and the noun 'speaker', where 'plain' meant 'clear, simple' and 'speaker' derived from 'speak' meaning 'one who speaks'.

The element 'plain' comes from Old French 'plain' and ultimately Latin 'planus' meaning 'flat, level, clear'. The verb 'speak' comes from Old English 'specan' (or 'sprecan'), which evolved into Modern English 'speak' and the agentive noun 'speaker'. The compound 'plain-speaker' is a modern English formation combining these elements.

Initially, 'plain' carried the sense of 'simple' or 'clear', and combined with 'speaker' it originally described someone who speaks clearly; over time the compound's emphasis shifted toward 'frankness' or blunt honesty, giving the current sense of 'someone who speaks plainly and candidly.'