Stralunatica is a fascinating Italian adjective that literally means 'moonstruck' or 'dazed'. It describes someone who is dreamy, absent-minded, or perhaps a bit eccentric, as if influenced by the moon.
In the song, the artist uses it to describe a love interest who is always 'with her head in the clouds' (perché hai sempre la testa fra le nuvole). It's a vivid and poetic word that paints a picture of a whimsical and captivating personality, making it much more interesting than a simple 'distracted'.
Bresh spends this whole song admitting he lost an argument with himself. "Non è che ho deciso io" (it's not like I decided it), he shrugs, and then hands over the reason: "Io t'avrei pure detto di no / Ma eri vestita da dio" (I would even have told you no, but you were dressed like a goddess). That last phrase is the title, and in Italian "da dio" is everyday slang for something done gorgeously well, so the compliment and the surrender arrive in the same breath. The chorus is where his arithmetic goes wrong: "Tu mi piaci ma / Tutti vorrebbero fare l'amore con te" (I like you but everyone would want to make love with you), followed by the very reasonable and very ignored conclusion "Forse ho bisogno di stare lontano da te" (maybe I need to stay away from you).
She gets the better lines. He calls her "Stralunatica", a word he built out of stralunata (dazed) and lunatica (moody), and complains that she saw him coming: "Mi hai sgamato, hai gli occhi dietro la schiena" (you caught me out, you have eyes in the back of your head). By the last verse he has given up on describing her in any order and simply lists: "Sei matta, sei seria / Sei finta, sei vera" (you're crazy, you're serious, you're fake, you're real). Then, because a contradiction that size needs a sound rather than a word, "Mi sembra un assolo di Clapton" (it feels like a Clapton solo).