E. De Felice, E. De Stefani and E. Rossoni agree that the surname Grassi originated as a medieval nickname derived from the Italian adjective grasso (“fat,” “stout,” or “well-built”), originally used to describe a person's physical appearance. Like many descriptive bynames of the Middle Ages, it later became a personal name and was eventually established as a hereditary surname. In medieval Latin documents, it is also found in the form Crassus.

Historical records attest to the surname’s early diffusion in northern Italy, particularly in Lombardy. One of the oldest known references is Landulfus Grassus, documented in 1191. During the 12th and 13th centuries, individuals bearing the surname appear in Milan, Monza and other Lombard centers, often holding important civic positions such as judges, consuls and military officers.

Over the centuries, several unrelated Grassi families developed independently in different parts of Italy. Some branches achieved considerable social prominence, acquiring lordships, feudal possessions and noble titles between the 14th and 18th centuries. This widespread and independent development confirms that the surname did not originate from a single ancestral line, but rather arose multiple times from the same descriptive nickname.

Today, Grassi is a pan-Italian surname found throughout the country. Its long history, extensive distribution and numerous independent family branches make it one of the classic examples of an Italian surname derived from a physical characteristic that became firmly established during the medieval period.