'firmly-set' originates from the combination of 'firmly' and 'set', where 'firmly' means 'in a secure manner' and 'set' means 'to place or fix in a position'.

'firmly' comes from Old English 'fyrmlice', and 'set' comes from Old English 'settan', both of which have retained their meanings over time.

Initially, 'firmly' meant 'securely' and 'set' meant 'to place', and these meanings have largely remained the same in modern usage.