The authors agree that the surname derives from the nickname grillo —the cricket, the insect of the same name—, which first circulated as a given name and later became a surname. E. De Felice documents this form as early as the 12th century: in Bari, in 1151, Nicolaus Grillus is recorded, and he notes that the surname Grillini is very frequent in Emilia-Romagna.

Surnames in Italy places the earliest evidence of the surname in the Monza and Brianza area: the Liber Consignationis Prebendarum of 1237 mentions "Beniamins, Pinamonte et Bregondus qui dicitur Grilli", which confirms that it was already working as a surname at that date. In the same period it appears in Vedano and Lissone. As a baptismal name, Grillus figures in the Milanese oath of obedience to the Holy See of 1266.

Surnames in Italy further specifies that the surname does not appear in the acts of the Municipality of Milan prior to 1216, nor in the lists of the Consiglio dei Novecento of the Golden Ambrosian Republic, but it does appear in lists of Monza merchants of 1326, 1350 and 1476. The historian Merati held that the surname would have a probable Ligurian ancestry.