In Spanish people say poner los cuernos (=to put horns) to define a situation in which somebody is cheating to theirN partner:
—¡Juan le ha puesto los cuernos a su novia! El otro día pasó la noche con una amiga.
—Juan y Antonia se separaron después de que se pusieran los cuernos mutuamente unas cuantas veces
That is, when somebody has had a relationship with another person (specially having sex) without his/her partner knowing, this expression is used.
Also, the DLE contains the meaning 'cheating' for cuernos:
- m. coloq. Infidelidad matrimonial. Marriage infidelity
I tried to find an explanation to this and found a site which details different hypothesis:
- mythological references to the god Mercury,
- Christian explanations on how this involves devil, who does have horns
but nothing seems to be definitive.
This concept seems to be quite widespread in Spanish culture, where a person to whom le han puesto los cuernos is a cornudo, for example.
Do you have proper references on the origin of this saying?