It was anciently believed that the eyes of some persons darted noxious rays on objects which they glared upon. The first morning glance of such eyes was certain destruction to man or beast, but the destruction was not unfrequently the result of emaciation. Virgil speaks of an evil eye making cattle lean. (See Mascotte, Jettator.)
“Nescio quis teneros oculus mihi fascinat agnos.” Ecl. iii. 103.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894