In this scene, the Scottish army is waiting to fight the English army. William Wallace (Mel Gibson) is going to “pick a fight” and make sure that the nobles from each army don’t negotiate a peace. His fellow leaders are in charge of passing out weapons. When asked what to do they remark, “we didn’t get dressed up for nothing.” 

They are falling prey to the sunk cost fallacy. Just because they’re all dressed and ready to fight doesn’t mean that is the logical thing to do. The soldiers are not using effective marginal analysis to determine whether fighting is the next best course of action.

Thanks to Luke Starkey for the clip and summary!