Jamás means "never" or "never ever." While it might seem like a common word, its repeated and emphatic use throughout the song makes it incredibly powerful and central to the message.
In "Jamás Te Dejaré," Rocio Durcal uses jamás to express an absolute and unwavering commitment. She sings "Jamás te dejaré" (I will never leave you) and "Jamás mi vida" (Never, my love), reinforcing a profound and eternal promise of love and loyalty, regardless of any circumstances. This strong declaration makes the word particularly memorable and impactful in the context of the song.
Rocío Dúrcal builds the whole song around one line, repeated until it works as the chorus: "Jamás te dejaré" [I will never leave you]. She doesn't leave it as a single promise. The first verse strips it down to nothing and the vow still holds: "Aunque no tengas pasado" [Even if you have no past], "Aunque no tengas presente" [Even if you have no present], "Aunque no tengas futuro" [Even if you have no future]. Take away a person's whole timeline and she's still there.
The second verse comes at the same promise from the opposite angle: it runs through everything that could be wrong with him instead. "Aunque fueras mentiroso" [Even if you were a liar], "Aunque fueras otras cosas" [Even if you were other things], "Aunque fueras lo que fueras" [Even if you were whatever you were]. None of it changes anything. "Y es que no hay nada que pueda separarnos" [And there's nothing that can separate us], she sings, and by the third time the chorus loops back around, it plays less like a promise and more like something she already decided a long time ago.