The meaning of the phraseological phrase "rub in the glasses" and the noun "eyewash" derived from it is associated with fraud, deception in order to obtain any benefit.
For a modern person, the word glasses, first of all, is associated with an optical device used by people suffering from myopia or farsightedness. Such glasses have to be wiped regularly, but it is quite difficult to imagine how they are "rubbed in", and why deceivers and fraudsters should do it.
Versions of origin
Some hypotheses explaining the origin of this phraseological unit really connect it with glasses - an optical device. There is an assumption that this expression was born simultaneously with the phraseological unit "Potemkin villages": allegedly Empress Catherine II did not notice that the "happy and abundant" villages that G. Potemkin showed her were fake, because her eyesight was poor, and even glasses were not allowed to see well. This explanation can be attributed more to the category of etymological myths than to the number of serious scientific theories.
Another hypothesis, also related to vision correction, inspires more confidence. In the 14th century, glasses were quite common in Europe. Demand gave birth to supply, and there were quite a few eyeglass manufacturers and dealers who had very little understanding of vision correction. They could neither correctly make nor select them for a specific person, but they were good at selling glasses to gullible customers - this was what was called "rubbing glasses".
But even this hypothesis does not explain why glasses are "rubbed in".
Points and gambling
The most convincing hypothesis seems to be that derives the phraseologism "rub glasses" from the jargon of gamblers. In this case, we are not talking about glasses - an optical device, but about a gambling card game called a "point". In this game, each card has a certain value, which is expressed in points: ace - 11 points, king - 4 points, queen - 3 points, jack - 2 points, the value of other cards was determined by the number of signs, which were also called points.
Dishonest players developed such "sleight of hand" that they managed to glue ("rub" into) an extra point sign to the card ("rub" into it) during the game, unnoticed by their partners. Thanks to this, the six, for example, turned into a seven, its value increased, helping the cheater to gain game balance dishonestly. This card fraud was called "rubbing glasses".
As is always the case with stable phrases, this turnover gradually lost its original meaning, acquiring a more general one - they began to denote any cheating when playing cards, not always associated with gluing unnecessary signs. Subsequently, fraud in general, including those not related to gambling, began to be called "eyewash".
