Specifically, to “swash a buckler” referred to the act of pounding a buckler (small shield) against one’s own chest as a sort of macho display.
Mmmm… close, but no cigar.
To “swash” is to make a noise by beating one’s sword against one’s shield (or against others’ swords). Via the OED (it’s definition #3 below):
And that “bucklers” are involved also tells you what kind of sword’s involved: not a big expensive longsword of one variety or another (for which you’d realistically need a way bigger shield for any kind of protection), but the shorter, lighter, cheaper weapons you might expect in back-alley street fighting among the (let’s all sneer decorously now) plebs and real-swordfighter-wannabes.
