In lunfardo has a curious and very old meaning. During the first half of the last century were very popular in Buenos Aires 'ladies' orchestras' which were made up of female performers (although not necessarily ladies) playing different styles on stage. And it turns out that not everyone knew music; there were always some to make numbers, which were pretty and were "sighted". But to justify their presence they gave them some violins, and they made the mimicry, as if they were playing. The method to prevent a sound from coming out by accident was to lather the strings of the instrument, so when they played them with the bow they no longer vibrated. These girls were known as the "soaps." And today they're just a memory.
