What does “poner los cuernos” mean in Spanish?

“Ponerle los cuernos a alguien” means to cheat on a partner — literally to “put the horns on” them. The betrayed person is the one wearing the horns: a “cornudo” (the cuckold). The image goes back centuries in Europe, where a man whose wife was unfaithful was pictured sprouting horns as a mark of public shame. It is the default everyday way to say cheat across Spain and Latin America.

Here the betrayal lands as “¿con razón no quería irse a vivir con mi mamá para ponerme los cuernos con Pablo?” — so that’s why she wouldn’t move in, so she could cheat on me with Pablo. Useful related forms: “cuerno” alone for the affair-tinged horn, “cornudo” for the cheated-on man, and the gesture of two raised fingers, which in this context is an insult, not “rock on.”