The pugio (Plural: Pugiones) 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. Like the gladius, the pugio was probably a stabbing weapon, the type said to have been preferred by the Romans.
Although it is impossible to be really sure, the word pugio possibly descends from the Proto-Indo-European root *peug-, "stab, stick." The root is the same as in English pugilist, "boxer." It is still possible to use punch and stab synonymously in many Indo-European languages; hence, Latin pugnus meaning “fist.”
The Smith article cited below proposes that the pugio was the weapon grasped by the fist; however, the Latin word for swordplay was pugna, an exchange of thrusts without the intermediary of fists, although it could also be a fistfight.
The pugio became an ornate sidearm of officers and dignitaries as well, a custom reminiscent of the knives after which the Saxons were named. These Germanic mercenaries served in the Roman army. The emperors came to wear a dagger to symbolize the power of life and death.
The emperor, Vitellius, attempts to resign his position and offers his dagger to the consul, but it is refused and Vitellius is forced to stay by popular acclaim and the Praetorian guard. Tacitus also relates that a centurion, Sempronius Densus, of the Praetorian guard drew a dagger to save Lucius Calpurnius Piso Licinianus momentarily.