Espinado is a poetic adjective that literally means "thorny" or "pricked by thorns," derived from the noun espina (thorn). It's a fantastic word that you won't hear in everyday conversation.
In this classic rock anthem, Santana uses it as a powerful metaphor. A 'corazón espinado' is a heart that has been hurt by love, feeling as if it's been pierced by thorns. This single word perfectly captures the song's central theme of agonizing heartache and unrequited love.
This is one of the great songs about a very specific kind of heartbreak. The title, "Corazón Espinado," literally means "a heart with thorns," and that's the central idea. The singer feels like a woman has physically wounded him, complaining, "Esa mujer me está matando / Me ha espinado el corazón" (That woman is killing me / She has stuck thorns in my heart). He doesn't just feel sad; he feels like his heart is "aplastado / Dolido y abandonado" (crushed / hurt and abandoned).
He keeps repeating how much everything hurts, from love itself to the act of forgetting. But he's not just feeling sorry for himself. He gets a little bit of revenge in by warning the woman, "no olvides mujer que algún día dirás / Cómo me duele el amor" (don't forget, woman, that one day you will say / How love hurts). He's convinced that because he was so devoted, she's bound to feel this same thorny pain eventually.