Inútil translates to "useless" or "futile." It's a strong word that conveys a sense of hopelessness or pointlessness, which isn't as common in everyday romantic song lyrics.
In "Nosso Adeus," Tim Maia uses it in the line "Inútil te dizer" (Useless to tell you). This suggests that words are insufficient or won't change the situation, adding a layer of poignant resignation to the song's theme of a temporary goodbye. It makes the listener wonder what exactly is so futile to say.