'mirror-symmetric' is a compound formed from English 'mirror' and the adjective 'symmetric'; 'mirror' originates from Old French 'mirour' (from Medieval Latin 'mirare') where the root related to looking/seeing, and 'symmetric' ultimately comes from Greek 'symmetria' via Latin/French where 'sym-' meant 'together' and 'metron' meant 'measure'.

'mirror' entered Middle English as 'mirour' from Old French and developed into modern English 'mirror'; 'symmetric' came into English from Latin/Old French forms of Greek 'symmetria' (and French 'symétrique'), and the compound adjective 'mirror-symmetric' was formed in modern English to describe reflection symmetry.

Initially 'mirror' referred to an object for looking or reflecting and 'symmetric' to balanced proportions ('measured together'); over time the compound came to mean specifically 'matching one's mirror image' or 'having reflection symmetry'.