cheating on someone
Mary le puso los cuernos a su marido.
Mary cheated on her husband.
She cheated on me.
Seguro tenía un auto con esos techos que se abren para que salgan los cuernos.
He probably had a car with one of those roofs that open up so the horns stick out.
— El secreto de sus ojos (2009)
¿Alguna vez has pensado que tu novia te estaba poniendo los cuernos?
Have you ever thought your girlfriend was cheating on you?
He cheated on me.
And these are the horns.