Fundiste comes from the verb fundir, which literally means "to melt" or "to fuse". However, in a more colloquial and passionate context, it can mean "to blow someone's mind" or "to overwhelm someone with intense emotion or pleasure".
In this song, the lyric "Esa noche me fundiste" (That night you melted me / you blew my mind) is used to describe an incredibly intense and unforgettable night of passion. It conveys a sense of being completely consumed and overwhelmed by the experience, making it a vivid and enticing word choice.
The whole song is one guy's alibi. He keeps repeating No fue que te busqué (it wasn't that I looked for you) and then, as if that settles it, Pero llegaste sola (but you arrived alone). Nobody chased anybody. She just happened to walk in.
The best bit is the timing. Del party me iba / Tú estabas llegando (I was leaving the party / you were arriving) puts the two of them passing each other in a doorway, and everything after that is him losing the argument he started. He was fine, and then Tú a mí me partiste (you broke me), and by the second verse he is writing up paperwork: Mami, te hice un contrato (baby, I made you a contract), daily, at twelve. Harryson's verse swerves off to settle a score with imitators, Y que quieren parecerse a nosotros (and they want to look like us), which is a very reparto thing to do halfway through a love song.