Arrescostada is a less common, more informal way of saying "recostada," which means "leaning" or "resting." It adds a touch of intimacy and colloquialism to the lyrics.
In the song, the singer imagines his love interest "arrescostada sobre mi pecho durmiendo" (leaning on my chest sleeping). This word choice paints a vivid and tender picture of closeness and comfort, making it a memorable and emotionally resonant word in the context of the song's longing.
Have you ever been completely heartbroken, while your ex seems to be doing just fine? That's exactly what Wise The Gold Pen is singing about in "Ay Ay Ay"! His friends tell him his ex isn't thinking about him anymore, and that she's doing great without him. He's heard she's even "sacaste del corazón" [taken him out of her heart].
Meanwhile, he's in agony, saying "mi vida es un infierno sin ti ay, ay, ay" [my life is a hell without you ay, ay, ay] and that it's so hard to get used to life without her. He can barely survive! He keeps imagining her next to him, sleeping on his chest, or kissing him, but then he's hit with the reality that she's gone. He even looks at his phone hoping she'll call, but nothing. He misses her so much, but she acts "como si nada" [as if nothing happened]. It's a really relatable feeling when you're stuck in the pain of a breakup and the other person has moved on.