Presentimiento translates to "hunch" or "premonition." It's a word that evokes a sense of intuition or a feeling about the future, often with a touch of drama.
In the song, Alejandro Santamaria sings, "Tengo el presentimiento que los esfuerzos por enamorarte nunca le sirvieron." He uses this word to express his gut feeling that another person's attempts to win over his love interest were unsuccessful, adding a layer of emotional foresight to the lyrics.
Alejandro Santamaria spends "Ni Una Palabra" arguing himself out of staying quiet. He is done pretending, singing "No me interesa ya disimular" (I do not care to hide it anymore) about what he still feels for someone who is already moving on. He knows the timing is bad, maybe she is happy without him, but he would rather risk saying it than swallow it again: "Prefiero el intento, a quedarme en silencio" (I would rather try than stay silent).
The title is the thing he is afraid of repeating, losing her again "sin decir nada" (without saying a word), "ni una palabra" (not one word). He is clear eyed about the guy replacing him too, admitting "Le falta una parte y esa yo la tengo" (he is missing a part, and I have it), not as an insult but as the one thing he is sure of. So instead of standing back and letting it happen quietly again, he says something this time.