'plainly-constructed' originates from the combination of 'plainly' and 'constructed'. 'Plainly' comes from the word 'plain', which originates from Latin 'planus', meaning 'flat, level'. 'Constructed' comes from Latin 'constructus', the past participle of 'construere', meaning 'to heap together, build'.

'plainly' evolved from Middle English 'plein', and 'constructed' evolved from Middle English 'construen', both eventually forming the modern English term 'plainly-constructed'.

Initially, 'plainly' meant 'clearly' or 'evidently', and 'constructed' meant 'built'. Together, they evolved to describe something built in a straightforward manner.