Pugio Dagger

Pugio:

The pugio was a dagger used by Roman soldiers as a sidearm. It seems likely that the pugio was intended as an auxiliary weapon, but its exact purpose to the soldier remains unknown for sure. 

Attempts to identify it as a utility knife are misguided as the form of the pugio is not suited to this purpose and in any case utility knives of a variety of sizes are common finds on Roman military sites, meaning there would be no need for a pugio to be used in this way.

Officials of the empire took to wearing ornate daggers in the performance of their offices, and some would wear concealed daggers as a defense against contingencies. The dagger was a common weapon of assassination and suicide; for example, the conspirators who stabbed Julius Caesar used pugiones.

(Taken from wikipedia)