Leonard wants to be a "swashbuckler," but that term is more correctly applied to a slashing weapon, such as the saber. The guys are holding fencing foils, which are stabbing weapons.
He is clearly referring to a shipmate or pirate who always had swords. It's just a throwaway line meant for humor.
In the fifth season, Sheldon and Kripke compete for an office using sports as an equalizer since both are such bad athletes, yet now Barry is not only a fencer, but so proficient at it that he can teach others.
The guys are wearing their fencing jackets incorrectly as the strap (croissard) that passes between their legs is too tight. If they made a lunge, it would be very painful. This can't have been done for comedic effect as it is far too subtle to catch for anyone who is not highly familiar with fencing, and it never comes up, they never learn the hard way and get hurt.
When trying to figure out Sheldon's dominant leg, Howard asks how he flushes public toilets, with the idea that it will be his dominant leg. When facing a standard toilet, such as public toilets, the flush handle is always on the left side. Anyone inclined on performing the awkward maneuver of flushing with their feet would do so with their left, as it's on the same side of the toilet, instead of trying to cross over with the right while also standing on one foot, the left. It would have nothing to do with dominant legs, and it makes no sense that Sheldon would answer "right".