Greñas is a colloquial Spanish word for 'shaggy' or 'messy hair'. It's a more informal and expressive alternative to the standard words pelo or cabello.
In the song, Ivan Cornejo sings the powerful line, "Arrancas mis greñas cuando pienso en recuerdos" (You make me want to rip my hair out when I think of memories). This is a visceral metaphor for extreme frustration and mental anguish, vividly capturing the feeling of being so overwhelmed by painful memories that you want to physically pull your hair out.
Ivan Cornejo writes some of the most dramatically sad breakup tracks you will ever hear, and "Ya Te Perdí" is no exception. He is spiraling after a split, calling an ex way too late with the line "Discúlpame, bebé / Por molestarte, sé que es tarde" ("Forgive me, baby, for bothering you, I know it's late"). He even begs her to give him his joy back: "Regrésalo pa' atrás / Mi felicidad" ("Bring it back, my happiness"). His weekends are ruined, and he has to face the fact that she has moved on to someone new with a romance that turned out to be totally flimsy: "Tu amor fue hecho de papel" ("Your love was made of paper").
The sting comes from planning a whole life with someone who did not plan one with you. As he puts it, "Lo que más duele / Es que yo te vi / En mi futuro / Pero yo no en el tuyo" ("What hurts the most is that I saw you in my future, but not me in yours"). To show how messy the grief feels, he drops a great visual idiom: "Arrancas mis greñas" (literally "You tear out my long/tangled hair"). In everyday Spanish, greñas refers to messy or uncombed hair, so the phrase shows him clutching his head in total frustration when the memories hit. It is over-the-top, dramatic, and brutally honest about getting left behind.