Pupetta is a charming and affectionate diminutive of pupa, meaning 'doll'. It's a term of endearment, similar to 'sweetheart' or 'baby doll' in English.
In "Pronto Pronto," Adriano Celentano uses pupetta to address the woman he's calling, immediately establishing a playful and tender tone. It's a less common, more endearing term than a simple 'cara' (dear), making it memorable and unique to the song's flirtatious charm.
This whole song is basically one side of a very energetic phone call! Adriano Celentano is trying to convince a girl to go out with him, and he is not taking no for an answer. He starts by asking, "Pronto pronto, pupetta, stasera che fai?" (Hello hello, little doll, what are you doing tonight?). He's full of promises for a wild night of dancing.
He tells her they'll do "tre salti" (three jumps) and that he wants to "per aria ti voglio gettar" (throw you in the air). It sounds like some seriously acrobatic dancing! To seal the deal, he promises intense devotion, saying he'll hold her, kiss her, and even "ti urlerò / Che solo tuo, tutto tuo, sempre tuo sarò" (shout to you / that only yours, all yours, always yours I'll be). It's a fun, slightly over-the-top declaration of passion, all to get a date for the evening.