I’m not talking about what holds the ends of your belt together. You might buckle with your buckle to buckle, but that’s not very Shakespearean, is it?
buckle (v.) IPA Pronunciation: 
grapple, fight, combat
CHARLES
In single combat though shalt buckle with me.
– Henry VI, part 1 (IV.iv)
Here, Charles the King of France is challenging Joan of Arc so that she may prove she has divine support before letting her lead his men into battle. It’s always good to test that sort of thing instead of taking their word for it.
This word’s meaning can usually deduced from the context. But don’t get confused, buckle is also used with the definition you’re more familiar with.