Frosini is a typically Tuscan surname, of the areas between Pisa, Pistoia, Prato and Florence; it derives from the medieval name Frosino. Frosino originates from the hypocoristic aphaeresis of the name Eufrosino, which derives from the ancient Greek Ευφροσυνη (Euphrosyne). Eufrosino means "joy", "gladness", "happiness", where Eu = "good" + phren = "mind", "heart".

• Frosi is more typical of Cremona
• Frosio of Bergamo and Brescia
• Frusin, almost unique and typical of Como

Other variants: Frosini, Frosinini, Frosina, Frosinoni. In other cases it could derive from the Lazio place name Frusino, an old Latin form of the present name Frosinone, an Italian province and city. In that case, the surname would indicate that its first bearers came from that region.