clavus militis

hobnail boot
Soldier's Hobnail Sandal (photo courtesy of VRoma)

Footsoldiers and centurians wore a heavy sandal-like shoe, the caliga, that consisted of a thick sole studded on the bottom with nails; it was bound to the foot and ankle with leather straps.
Umbricius here identifies the military boot by its most prominent feature, the clavus or nail that made long marches over difficult terrain possible and served as a weapon against the shoeless.
Gaius Caesar Germanicus, the Emperor Caligula, was given his nickname by the troops because of the tiny boots of the common soldier that he wore as a child on campaign in Germany with his father (Suetonius, Vita Gai 9).

Soldier's Hobnail half-sandal (photo courtesy of VRoma)