Mentiría is the conditional form of the verb mentir (to lie), meaning "I would lie". It's a powerful word because it sets up a hypothetical situation, hinting at a truth the singer is reluctant to admit.
In the song, the artist sings, "Te mentiría si no te digo que a tus brazos de nuevo volvería" (I would be lying to you if I didn't tell you that I would return to your arms). This reveals a deep, hidden desire, making the word incredibly enticing and central to the song's emotional conflict.
This song is all about a guy who is trying so hard to pretend he's over his ex. He starts off by telling her he's not looking for a girlfriend right now, saying, "Que ya no estoy para novias" [I'm not up for girlfriends anymore]. He insists he's doing great, enjoying "La noche, los tragos, la gira" [The night, the drinks, the tour]. But the whole act falls apart the second he sees her. He immediately admits, "Pero pasas enfrente y siento que muero" [But you walk by in front of me and I feel like I'm dying].
The whole chorus is him confessing the truth. He says, "Te mentiría / Si no te digo que a tus brazos de nuevo volvería" [I'd be lying / If I didn't say I'd return to your arms]. He even says something pretty possessive: that she can be with another guy as long as she's still his. It's a classic case of someone whose pride is fighting with their heart, all set to an incredibly catchy Cuarteto beat that makes you want to dance through the drama.