Pifias comes from the verb pifiar, which is a colloquial term meaning "to mess up" or "to screw up". It's a less formal and more colorful word than equivocarse (to make a mistake).
In the song, the singer uses pifias to express his frustration and disappointment when he realizes his partner has been unfaithful, despite her assurances of exclusivity. He sings, "después de comprobar que tú me pifias cuando me decías que era sólo mías" (after confirming that you mess up when you told me I was only yours), highlighting the betrayal in a very direct and impactful way.
Have you ever suspected someone was cheating, and then found out the hard way? That's exactly what happens in Antonio Ríos's "Ayer Te Segui Los Pasos"! Our singer followed his love interest yesterday, and what he saw broke his heart. He sings, "pude ver con mis propios ojos y entregar y otros brazos" [I could see with my own eyes and give myself to other arms]. Ouch!
He's realizing the truth about this person's "falsedad" [falsehood] and how they lied when they said he was "sólo mía" [only mine]. He repeats this feeling of betrayal, saying "me di cuenta de todo de tu falsedad" [I realized everything about your falsehood] and then, simply, "y me puse a llorar" [and I started to cry]. It's a classic cumbia with a truly heartbreaking story about discovering a partner's infidelity.