Ingrata translates to "ungrateful woman." It's a strong and somewhat dramatic term used to describe someone who doesn't appreciate what they have or what others do for them.
In this song, the artist uses ingrata to express his frustration and disappointment with women who don't value the efforts and love of men, particularly husbands. It's a key word that captures the central theme of the song: the perceived lack of appreciation from women towards men.
Saul El Jaguar Alarcón's "Sinonimos De Marido" is a really interesting take on what it means to be a husband, especially when you feel unappreciated. He sings about the husband as "el eterno enamorado de la vida, el eterno enamorado del amor" [the eternal lover of life, the eternal lover of love], a guy who always gives his best, even when he's treated poorly, "como si fuera un pelele, como un títere sin alma y sin razón" [like a puppet, like a soulless and senseless puppet]. It's a bit heartbreaking because he can't understand why women would treat someone they supposedly love this way, even saying, "me pega por entero el corazón" [it hits my heart completely].
He really lays into the idea that women are "mal agradecidas" [ungrateful] and bring men "penas y dolor" [sorrows and pain]. He sees the husband as a "valiente que es cortés y muy decente" [brave man who is courteous and very decent], someone who's like "un artista incomprendido, un tesoro en el olvido, un talento que jamás se aprovechó" [an misunderstood artist, a forgotten treasure, a talent never taken advantage of]. The song suggests that a husband is like "un diamante más pulido" [a more polished diamond], but often, the woman just doesn't get it. It's a strong, almost frustrated, look at the sacrifices and often unacknowledged efforts of a husband.