It was originally known just as Corpi Santi; "di Milano" was added in 1859, possibly to avoid confusion with the comune with the same name located in the area of Pavia.
A more mundane explanation for the toponymy is that, during Austrian rule, sanitary laws were enforced whereby the Milanese would have to bury their dead outside the city walls.
[1] The establishment of the comune of Corpi Santi di Milano was decided by Empress Maria Theresa in 1757 and actuated by her son Joseph II in 1782.
Corpi Santi had an area of about 66 km2[2] and it was roughly ring-shaped, centered on Milan, and extending for 6–7 km from the city walls outwards.
Its economy of course depended on Milan; being so close to the city walls, farmers in Corpi Santi could dedicate to cultivating perishable but profitable vegetables such as onion, cabbage, fruit.