EfĂmero translates to "ephemeral" or "short-lived." It's a beautiful and somewhat less common word that evokes a sense of fleetingness and impermanence.
In the song, Paty CantĂş uses it to describe a love story that is "como ese cuento efĂmero que encuentra su final" (like that ephemeral story that finds its end). This word perfectly captures the song's theme of skepticism towards eternal love and the desire for something real, even if it's not forever.
Paty Cantú opens with a countdown: "Si dices que me amas / Tienes un minuto y nada mas" (If you say you love me / You have one minute and nothing more). No more chances for words that don't hold up - "No digas palabras que / Después no cumplirás" (Don't say words that / Later you won't fulfill) - she's already paid for that mistake once: "Ya he sufrido demasiado" (I've already suffered too much), and she warns "Si te creo y no me amas / Te odiaré, te hare sufrir" (If I believe you and you don't love me / I'll hate you, I'll make you suffer). The chorus lands on the contradiction driving the whole song: "Quiero todo y quiero nada" (I want everything and I want nothing).
What she wants instead of a fizzling romance is one that keeps shifting shape - "Que hoy sea explosiva / Que mañana sea pasiva" (That today it be explosive / That tomorrow it be passive) - anything but "la rutina" wearing it down. She'd rather it stay unpredictable for good than settle into the kind of story that runs out after a few pages: "Una historia de tres paginas / Como ese cuento efĂmero" (A three-page story / Like that ephemeral tale).