Mentiría means "I would lie" and comes from the verb mentir (to lie). It's a key word in the song, as the title itself is "Te Mentiría" (I Would Lie to You).
In the lyrics, Luck Ra uses this word to express a deep, undeniable truth. He sings, "Que te mentiría / Si no te digo que a tus brazos de nuevo volvería" (That I would be lying to you / If I didn't tell you that I would return to your arms again). It highlights the internal conflict and honesty about his lingering feelings, making it a powerful and central theme of the song.
This song is a classic case of someone trying so hard to pretend they're over their ex, but failing completely. Luck Ra starts off by saying he's not looking for a girlfriend—"Que ya no estoy para novias" (I'm not up for girlfriends anymore)—and then brags about how great his life is now, with all the parties, tours, and money. But the whole tough-guy act falls apart in the very next line when he admits, "Pero pasas enfrente y siento que muero" (But you walk by and I feel like I'm dying).
The whole song is built around this big lie he's telling himself and everyone else. The chorus is a direct confession: "Que te mentiría / Si no te digo que a tus brazos de nuevo volvería" (I'd be lying / If I didn't tell you I'd return to your arms again). He even gets a little possessive, saying she's still his even if she's with other guys. He then describes her new boyfriend as the exact opposite of him—a calm, well-dressed guy who doesn't get drunk or have problems with the police—and sarcastically asks, "¿No era eso lo que querías?" (Isn't that what you wanted?). It's a fun, brutally honest song about wanting someone back, even when you know you're probably not the 'right' person for them.