Testamento literally means "will" or "testament," the legal document for your belongings after death. It's a very dramatic and unusual word to find in a pop song!
Becky G uses it metaphorically to express the depth of her heartbreak. She sings, "El día que terminamos firmé mi testamento" (The day we broke up, I signed my will), to show that the breakup felt like a death and that her heart is now "muerto en vida" (dead while alive).
Have you ever looked back at an ex and thought, “Man, we were so close”? That is the exact spot Becky G and Oscar Ortiz drop us into with "BESÁNDOTE." Right out of the gate, Becky hits you with "Casi logramos que funcionara esto / Casi, pero no" ("We almost made this work / Almost, but no"). She even goes full drama queen, admitting that the breakup felt like drafting her last will: "firmé mi testamento / Porque está muerto en vida ya mi corazón" ("I signed my will / Because my heart is a living corpse"). It is blunt, slightly overdramatic, and hurts because she gave a full 100% and still watched things fall apart without knowing why.
The real punchline of the track comes in the chorus, where regret takes over. They both admit that if they could turn back the clock, they would hold on tight, drop their pride, and keep one specific moment going forever: "Si hubiera sabido que era el último beso / Todavía estaría besándote" ("If I had known it was the last kiss / I would still be kissing you"). It turns an ordinary goodbye into an endless freeze-frame. Oscar Ortiz joins in to double down on that stubborn longing, swearing he will hold onto hope even if ten thousand years pass ("pasen más de diez mil años"). It makes for a sweet, painful story about the agony of knowing that the last kiss was, well, the last one.