Cornisa literally means "ledge" or "cornice", referring to the decorative molding at the top of a building. It's a specific architectural term you don't often find in song lyrics.
In the song, Gustavo Cerati sings "Estamos al borde de la cornisa, casi a punto de caer" (We're on the edge of the ledge, almost about to fall). This creates a powerful, cinematic image of tension and anticipation, putting the characters in a risky, high-stakes situation that perfectly captures the thrill and suspense of the song's voyeuristic theme.
Soda Stereo brings pure 80s new-wave rock energy in this track, telling a story straight out of a noir film. The narrator casts himself as a voyeur watching someone undress from behind window blinds: "Soy un espía, un espectador" [I am a spy, a spectator]. Instead of wanting a normal, close-up romance, he explicitly prefers keeping his distance. He wants her "Fuera de foco / Inalcanzable" [Out of focus / Unreachable] and "Casi intocable" [Almost untouchable]. The title itself refers to venetian blinds—a "persiana americana"—the exact barrier he hides behind while staring through the slats.
There is a thrilling game of mutual temptation playing out here. He suspects she knows she is being watched and actually enjoys the thrill, singing "Sé que te excita pensar hasta donde llegaré" [I know it excites you to think how far I'll go]. The whole scenario feels risky and intense, perched "al borde de la cornisa / Casi a punto de caer" [on the edge of the ledge / Almost about to fall]. He calls the suspense "una condena agradable" [a pleasant sentence/condemnation], turning a voyeuristic secret into a weirdly addictive ritual that he refuses to give up.