Terca translates to "stubborn" or "headstrong." It's a direct and somewhat informal way to describe someone who is unwilling to change their mind or accept a situation.
In this song, Christian Nodal uses terca to tell his ex-lover to stop insisting on returning or making demands. He's essentially saying, "Don't be stubborn; it's over, and there's no going back." It's a common word in everyday Spanish but used here with a strong, dismissive tone that makes it memorable in the context of a breakup.
Christian Nodal's "La Que Se Fue, Se Fue" is a sassy, no-nonsense anthem about moving on from a past relationship. Our guy is completely over it, telling his ex, "Usted se despidió solita" [You said goodbye all by yourself]. He makes it clear that once she left, she lost all rights to regret it. He's not holding back, asking, "¿Con qué cara vienes y me reclamas que ya tengo otra en mi cama?" [With what face do you come and complain that I already have someone else in my bed?], especially since she supposedly didn't love him anymore and didn't want to know anything about him.
The main message is loud and clear: "La que se fue, se fue y no puede volver" [The one who left, left and can't come back]. He's taken back all the kisses that used to be hers and firmly states, "reversa aquí no hay" [there's no reverse here]. He's only looking forward, "puro pa' delante y no pienso frenar" [only forward and I don't plan to stop]. He even tells her to "Llégale y sin llorar" [Get going and without crying], because he's not interested in any drama. It's a blunt, funny, and very direct song about closing a chapter and not looking back!