'plain-spoken' originates from the combination of 'plain' and 'spoken', where 'plain' meant 'clear' or 'simple' and 'spoken' referred to 'expressed in words'.

'plain-spoken' evolved from the Middle English word 'pleyn' meaning 'simple' and 'spoken', which has remained largely unchanged.

Initially, it meant 'expressed in simple words', but over time it evolved into its current meaning of 'expressing opinions in a straightforward manner'.