1886, "single eyeglass," from Fr. monocle, from L.L. monoculus "one-eyed," from Gk. monos "single, alone" + L. oculus "eye" (see eye).
"That this, a hybrid, a Gallicism, and a word with no obvious meaning to the Englishman who hears it for the first time, should have ousted the entirely satisfactory eyeglass is a melancholy illustration of the popular taste in language." [Fowler]