Arrepiento comes from the verb arrepentirse, meaning "to regret" or "to repent". It's a strong emotional verb that conveys a sense of remorse or a change of heart.
In "UNSEND", Becky G uses it to declare, "Que no me arrepiento de los besos que te di en la boca" (That I don't regret the kisses I gave you on the mouth). This word is enticing because it immediately signals a moment of bold confession and defiance, making listeners curious about the story behind her lack of regret.
The song opens on a habit rather than a feeling: "Me encanta cuando envías mensajes / Y los borras" ("I love it when you send messages and you delete them"). He writes, thinks better of it, and takes it back — and she has decided to say out loud the thing he keeps unsending. What she asks for first is the evidence: "Mándame / Las fotos mías / Cuando te quite la gorra" ("Send me the photos of me, when I take your cap off").
Then the admission, addressed to him as a friend and taking that word apart in the same breath: "Amigo mío, quiero decirte / Que me tienes loca" ("My friend, I want to tell you that you're driving me crazy"), and "Que no me arrepiento de los besos / Que te di en la boca" ("that I don't regret the kisses I gave you on the mouth"). She hands herself an excuse — "Se mezclaron las ganas que tenía / Con un par de copas" ("the wanting I already had got mixed with a couple of drinks") — and then takes it back a line later: "Y lo hice apropósito porque quería" ("and I did it on purpose because I wanted to"). The chorus is the actual request: "Hoy no quiero que sea un TBT / Yo quiero que seas mi bebé" ("today I don't want it to be a TBT — I want you to be my baby"). TBT is Throwback Thursday, the tag people put on something that already belongs to the past. She wants the opposite, and she wants him to stop deleting.