'mirror-image' is a compound word formed from 'mirror' and 'image', where 'mirror' comes from Old French 'mirour' meaning 'reflecting glass', and 'image' comes from Latin 'imago' meaning 'likeness' or 'copy'.

'mirror-image' was formed in modern English by combining 'mirror' and 'image', both of which have roots in Old French and Latin, and eventually became the modern English term 'mirror-image'.

Initially, it referred specifically to the reflection seen in a mirror, but over time it evolved to mean any reversed or symmetrical counterpart, not just literal reflections.