Sprofonderò comes from the verb sprofonare, meaning "to sink" or "to plunge deeply". It's a dramatic and evocative word that isn't used in everyday conversation as frequently as simpler verbs.
In the context of Bugo's song, the line "Se sprofonderò" (If I will sink/plunge) conveys a sense of uncertainty and vulnerability about the future of the relationship. It suggests a fear of falling into despair or a deep emotional abyss, making it a powerful and memorable choice.