Apparently this is a very common expression in Spain (which is unfortunate):
Poner los cuernos.
Literally, it translates to “Put (on) the horns.” The expression is used to signify that someone in a relationship has been unfaithful to their partner. More casually in English we would say that the person cheated on the other. In old English, it was common to say that a person was being cuckolded.
To conjugate in context, here are some examples:
“Su novio le ponía los cuernos.“
Her boyfriend was cheating on him.
“¡No me pongas los cuernos!”
Don’t cheat on me!
It is also common in Spanish to say:
Engañarle a alguien.
The above translates to “to fool someone” and often signifies an unfaithful partner. I think I prefer the horns, however, because it gives you the image of a devil (at least for me). In context, here are some conjugations:
“Ella me engañó.”
She cheated on me.
“Él no sabía que ella lo engañaba.”
He didn’t know that she was cheating on him.