I was teaching a workshop on solving cryptic crossword clues last month and someone asked me about a clue she was having trouble with in her regular newspaper, Rot found in English trees (7). The answer is eyewash (E = English, + yew + ash). But why does eyewash mean rot or nonsense?
The first meaning, dating from the early 18th century, is a medicinal liquid to wash out the eyes, a variation on the already-existing eye-water. Then, in the mid-19th-century it began to be used by the British army in India to refer to words, behaviour or activities that were not genuinely meant, but intended for outward show. By the late 19th century the meaning had widened to cover any sort of empty talk, humbug or nonsense, perhaps by association with the word hogwash.
The word eye on its own had a connection with stuff and nonsense before eyewash was used in this sense. The first citation for all my eye in the OED is from 1763, and the first mention of all my eye and Betty Martin in the OED is from 1781. It is described at this time as 'a sea phrase'. Variations on this idiom include all my eye and (my) elbow, all my eye and my grandmother, and all my eye and Tommy.