The pugio is fighting dagger of the Roman soldier.
This weapon has been copied directly from the Hispanic models and was not part of the panoply of the Legions before the Spanish expedition of Scipio during the Second Punic War.
The pugio was not worn by all Legionaries, but was a sign of wealth for the wearer. From the first models, some are richly inlaid with silver, and even coral like the model presented here.
The use of secondary weapons seems obvious to ensure one survival in delicate combat situations when the soldier has lost or damaged his main weapon, but it obviously may have other uses. It can be used to slaughter cattle, and cutting meat. It may be more useful than a gladius in confined spaces as during the attack of some villages or certain cities houses were space is scarce.
Then may be far from a secondary weapon, it should be considered more as a special tool dedicated to those situations when the Gladius is not suitable.
A late republican fresco shows a centurion carrying it on the front of its stomach in an unusual position. It is in fact a very appropriate way of carrying it in a street situation.
For instance it happened to me a couple of times in the middle of the crowd that someone try to remove my pugio which located on the side in its normal position can easily be stolen inside a compact crowd. Some reenactors Legionaries indeed do not hesitate to attach their pugio with leather laces to avoid this. Therefore this position not only avoids this inconvenience, but better if someone cincture you, you can still catch it in this position, while if it is on the side is impossible.
Experimentation on a fine wool undress tunic. The test proves very conclusive, the position is extremely practical and comfortable. If it tried in the vertical position at this places, one can imediatelly see that it is very uncomfortable.
Warning, do not try this with a cheap pugio that could slide out of it’s scabbard too easily and be quite dangerous to your own anatomy!