E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
From the Greek ops (the eye). Considered unlucky for the same reason that peacocks feathers in a house are said to be unlucky. A peacocks feather, being full of eyes, act as spies in a house, prying into ones privacy. Similarly, it is unlucky to introduce the eye-stone or opal into a house, because it will interfere with the sanctity of domestic privacy. (See CERAUNIUM).