Toco fondo is a powerful Spanish idiom that literally translates to "I touch the bottom".
Figuratively, it means "to hit rock bottom". In this passionate bachata, Romeo Santos sings "Por ti toco fondo" to express the ultimate sacrifice. He's willing to go to the lowest possible point, to lose everything, for the person he loves, showing the endless and unconditional nature of his feelings.
When Romeo Santos and Justin Timberlake join forces on a smooth Bachata beat, they bring pure devotion to the table. In "Sin Fin", the message is totally direct: this love has no end point, and neither singer is going to let it slip away. They make it clear that no matter what obstacle pops up, they are willing to put everything on the line. Romeo sings about hitting rock bottom and losing his voice just fighting for this relationship—literally crying out that he is "Agotado, agitado, sin voz" ("Exhausted, agitated, without a voice"). He even uses the Spanish idiom "toco fondo" ("Por ti toco fondo" ["For you I hit rock bottom"]), which means reaching the absolute lowest point possible for someone else's sake.
Justin matches that extreme energy in English, promising that even if his partner walks out the door, his love will follow right behind them. The track mixes Spanish and English back and forth as both singers declare that their heart's main job from God is simply beating for their lover ("Dios me dio este corazón / Y su prioridad es latir por ti" ["God gave me this heart / And its priority is to beat for you"]). Between dramatic promises to go to war and romantic sweet talk calling her "mi reina, mi luz" ("my queen, my light"), the song turns absolute romantic desperation into an irresistible dance track.