A Swashbuckler isn't someone who buckles his swash; he's someone who swashes his buckle(r).
If you aren't up to speed on archaic weapon combinations, the buckler is a small shield held in the hand or fist, able to be moved rapidly to block blows or to punch at an opponent. You can find an introduction to sword and buckler fighting on this page; inevitably there's some sword and buckler fencing on youtube too.
To swash it is to beat it with your sword hilt. It's the drum noise you get at the start of this trailer for Zulu except the shields are cowhide and they're hitting them with the butt of their assegais, rather than swords (can't find more relevant video at the moment).