Me arrepiento means 'I regret' and comes from the reflexive verb arrepentirse (to regret). It's a powerful and emotional phrase you'll hear often in dramatic Spanish songs and movies.
In 'Si Una Vez,' the singer repeats this phrase in the chorus, turning a past declaration of love completely on its head. He sings, 'Si una vez dije que te amaba, hoy me arrepiento' (If I once said I loved you, today I regret it), making it the passionate and defiant heart of the song.
This song is the ultimate anthem for getting over someone who didn't deserve you. The singers are looking back at a past relationship and just shaking their heads, completely done with it. They admit they once said things like, "Si una vez dije que te amaba" (If I once said I loved you), but now they're taking it all back with a simple, powerful statement: "hoy me arrepiento" (today I regret it). They feel like they must have been out of their minds, saying "No sé lo que pensé, estaba loco" (I don't know what I was thinking, I was crazy).
They don't hold back on the details of how bad it was. They talk about putting themselves at their ex's feet only to be paid back with contempt. Now, the tables have turned. They're telling the ex to get lost and not even bother calling. Wisin's rap verse is especially direct, saying his love was like gold that was flushed down the toilet: "Mi amor es oro y tu lo tiras por el inodoro." It's a blunt and final goodbye, declaring that loving this person was a mistake and "Ese error es cosa de ayer" (That mistake is a thing of the past).