Corps-a-Corps is the fencing term for "running into the other guy". You're not supposed to do it, and it hurts quite a lot - which is ironic considering you've been hitting one another with swords all afternoon.

Often, Corps-a-Corps is a no fault action: you both ran into one another. A warning is given and nothing else comes of it. In other cases it can involve such cruelty as smacking your opponent upside the head with your bell guard - and in that case it carries stiffer penalties.