The fringed flap at the groin, dangling below the cuirass was called either an apron or pteruges, which sounds a little less effeminant. Romans didn't wear anything under their skirts, unless they were in a very cold clime (Britannia or northern Germania), where they might adopt the local trousers.

My reading indicates the the pteruges didn't come into even limited use in the Roman armies until the reforms of Marius among the officers (centurions and above to open) and Augustus among the soldiers. They weren't in general use until maybe 100AD, being optional before then. About the time they became generally issued, trousers became much more common, as they always had been amongest the auxialiary troops.

Previous to this, they just wore an above-the-knee tunic, and their armor came down nearly to the bottom hem of the tunic. The apron came into use as the armor shortened, to offer some limited protection for the family jewels.