Regalao' is a colloquial, shortened form of "regalado," which means "gifted" or "given away for free." It's a common slang term in Spanish, especially in casual speech and music.
In this song, the phrase "Ni regalao'" is used repeatedly to emphasize that the singer wouldn't want her ex-partner even if he were offered for free. It's a strong, dismissive statement that perfectly captures the song's theme of moving on and having no regrets.
This song is built around one line, repeated until it sticks: "Nadie te quiere ni regalo'" [nobody wants you, not even given away for free]. Paula Koops opens by clearing out useless things: a broom for sweeping the beach, a ring that doesn't fit. "¿Pa' qué quiero una escoba pa' barrer la playa?" [why would I want a broom to sweep the beach?] - and her ex belongs on that same list. She even puts him up for sale and gets no takers: "Te puse en oferta pero nadie te compra" [I put you up for sale but nobody's buying], then compares him to an opponent so easy that even Messi could dribble past him.
What she wants back is specific, not vague hurt: "Devuélveme todo el tiempo que yo te he da'o" [give me back all the time I gave you], the affection, the kisses. She's calmer without him around, and that's the part that bothers him most - she looks good, and he can't stand it. He wants to dim her; she won't stop shining. All he left her with is heartbreak and an ugly tattoo on her back. Not roses, not seven lifetimes, will bring her back to him. He's not worth taking even for free.