early 13c., from Old French mireoir "a reflecting glass, looking glass; observation, model, example," earlier miradoir (11c.), from mirer "look at" (oneself in a mirror), "observe, watch, contemplate," from Vulgar Latin *mirare "to look at," variant of Latin mirari "to wonder at, admire" (see miracle).
The Spanish cognate, mirador (from mirar "to look, look at, behold"), has come to mean "watch tower." Latin speculum "mirror" (or its Medieval Latin variant speglum) is the source of words for "mirror" in neighboring languages: Italian specchio, Spanish espejo, Old High German spiegal, German Spiegel, Dutch spiegel, Danish spejl, Swedish spegel. An ancient Germanic group of words for "mirror" is represented by Gothic skuggwa, Old Norse skuggsja, Old High German scucar, which are related to Old English scua "shade, shadow."
Words for 'mirror' are mostly from verbs for 'look', with a few words for 'shadow' or other sources. The common use of the word for the material 'glass' in the sense of 'mirror' seems to be peculiar to English. [Carl Darling Buck, "A Dictionary of Selected Synonyms in the Principal Indo-European Languages," 1949]
Figurative usage is attested from c. 1300. Used in divination since classical and biblical times; mirrors in modern England are the subject of at least 14 known superstitions, according to folklorists. Belief that breaking one brings bad luck is attested from 1777. Mirror ball attested from 1968.