Ancient Roman men's undergarments, or indumentum, consisted of a simple linen loincloth wrapped around the waist and worn day and night. This sublicaculum or licium, as it was called, was in early days the uniform for nobles as well as peasants, and a man's amictus, or outer garments,were flung over the man's bare chest. Later in the Republic, the tunica made an important addition to men's indumentum, but die-hard conservatives such as Cato kept up the old habits until the twilight of the Republic.
By Imperial times even peasants wore tunicae, and athletes were the only persons seen in public wearing only a loincloth.
Sources: History of Costume, Historic Costume for the Stage by Lucy Barton, Daily Life in Ancient Rome, Jerome Carcopino