'firmly-constructed' originates from the combination of 'firmly' and 'constructed'. 'Firmly' comes from the Old English 'fyrm', meaning 'strong', and 'constructed' from Latin 'constructus', meaning 'to build'.

'Firmly' evolved from the Old English 'fyrm' to the modern English 'firmly', and 'constructed' from Latin 'constructus' to the modern English 'constructed'.

Initially, 'firmly' meant 'strongly' and 'constructed' meant 'built'. The combined term 'firmly-constructed' has retained its meaning of being strongly built.