'plainspoken' is a compound of 'plain' and 'spoken'. 'plain' originates from Old French 'plain', specifically from Latin 'planus', where 'planus' meant 'flat, clear, obvious'. 'spoken' is the past participle of 'speak', from Old English 'sprecan'/'specan'.

'plain' entered Middle English from Old French 'plain' (from Latin 'planus'); 'speak' evolved from Old English 'sprecan'/'specan', with the past participle 'spoken' forming part of compounds. These elements combined in modern English to form the compound 'plainspoken' (recorded from the 18th–19th century in similar forms of 'plain-speaking' and 'plainspoken').

Initially the elements meant 'clear/obvious' (plain) and 'to utter words' (speak); over time the compound came to mean 'speaking in a clear, direct, and candid manner' rather than merely 'clear speech'.