A swash is a kind of belt. It has a large buckle. The archtype swashbuckler buckles his swash, all the while wielding a rapier in one hand and nothing in the other.

I would grant them a dodge bonus while wielding a weapon finesse weapon in one hand an nothing else. Maybe level/4 as a doge bonus.

Ether that or give them the abilities of a factotum of level/2. That may be an interesting synergy.