The authors agree that this surname derives from the medieval name and nickname Merlo, itself formed from the noun mèrlo ("blackbird") with two opposed figurative meanings: "foolish, naive" or "cunning, clever". E. De Felice documents the Latin form Merulus as early as Siena in the year 730, and notes that in the 12th century the name Merlo was very frequent in Genoa as a second name or surname. Both De Felice and F. De Laurentiis agree that the surname spread throughout Italy, with the greatest frequency in Liguria and in the north of the country.

E. De Stefani supplies a specific analysis for Friuli, where he proposes several complementary etymological routes. Besides the nickname derived from Merulus, he notes a possible toponymic origin linked to place names of Carnia —the mountainous area of northern Friuli, in the province of Udine— such as Merleit and the river Merlo. He also mentions as a secondary route a possible influence of the name Merlino, a figure of the medieval epic tales, a hypothesis that agrees with what R. Brondolo proposes independently.

De Stefani adds that the variant Merluzzi was formed as a diminutive of the old Friulian m(i)erlùz, derived from the Latin merulus with the suffix -uceus, and that in time that form became a surname. The surname is attested in Carnia from the 13th century. De Felice further records the variant Merlich in the Friuli-Venezia Giulia area.