Da mai is a poetic and paradoxical phrase that literally translates to "since never". It's a truly unique expression you won't find in a standard dictionary.
In the song, Gino Paoli sings, "Io ti conosco da sempre e ti amo da mai" (I've known you forever and I've loved you since never). He creates a beautiful contrast: a love that feels both ancient ("da sempre") and completely new, existing outside the normal flow of time. This phrase perfectly captures the magical, timeless feeling of falling in love at first sight.
This song is all about that magical, slightly unreal feeling of falling in love at first sight. Gino Paoli sings about seeing someone so beautiful he can't believe she'd notice him: "Non mi sembrava possibile che / Tra tanta gente che tu ti accorgessi di me" (It didn't seem possible that / among so many people you would notice me). The feeling is so intense it's like flying or being in a dream. He uses a beautiful, strange line to describe it: "Io ti conosco da sempre e ti amo da mai" (I've known you forever and I've loved you since never), which perfectly gets across how new and yet how fated this connection feels.
But there's a sad feeling running through it all. He knows this perfect moment can't last. He begs her to "Fai finta di non lasciarmi mai / Anche se dovrà finire prima o poi" (Pretend you'll never leave me / even if it has to end sooner or later). The whole song is a plea to stop time, to live inside this "long love story" just a little bit longer. He repeats the line "Ora è già tardi, ma è presto se tu te ne vai" (It's already late, but it's too soon if you leave), showing how desperate he is not to let the night, or the moment, end.