Farsante translates to "fake," "phony," or "impostor." It's a strong and emotionally charged word used to describe someone who is deceptive or not genuine.
In "PERREO TRISTE," Becky G uses this word to directly confront her ex-lover, singing, "No te lo mereces, eres un farsante" (You don't deserve it, you are a fake). This word perfectly captures the feeling of betrayal and disillusionment, making it a memorable and impactful choice in the song.
The title is the whole problem in two words. A perreo is a sweaty, grinding club dance, and Becky G spends this one crying through it. She is on the floor with somebody new, and the chorus says out loud what she is actually doing there: "Estoy perreando con él / Pensando que podríamos ser tú y yo" ("I'm grinding with him / thinking it could be you and me"). The new guy has done nothing wrong. "Aunque está loco conmigo / Y me da to' lo que pido" ("even though he's crazy about me and gives me everything I ask for") reads almost like an apology to him. It still does not work. "Cuando yo lo beso espero / Que me sepa a ti, y me sabe diferente" ("when I kiss him I hope he'll taste like you, and he tastes different") is the meanest line here, and she aims it at herself.
What keeps this off the sad-ballad shelf is how irritated she is. She opens by handing him the bill: "Pero nuestro futuro dañaste, jodiste" ("but you damaged our future, you screwed it up"), and by the last verse the manners are gone: "No te lo mereces, eres un farsante" ("you don't deserve it, you're a fake"). She mentions being drunk four separate times, and every chorus drops her back "Contra la pared" ("against the wall"), which is where a perreo happens and also where you end up when there is nowhere left to go.