E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
It is unlucky to break a looking-glass. The nature of the illluck varies; thus, if a maiden, she will never marry; if a married woman, it betokens a death, etc. This superstition arose from the use made of mirrors in former times by magicians. If in their operations the mirror used was broken, the magician was obliged to give over his operation, and the unlucky inquirer could receive no answer.
Looking-glass of Lao reflected the mind as well as the outward form. (Citizen of the World, xlv.)