A warpath is either the road to a battle or a hostile, aggressive mood. Note: there's probably going to be trouble when someone is on either warpath.
This was originally a Native American term for a real path: a warpath was the route taken on the way to a known or planned conflict. The warpath was the way to war. From there, this became a general term for being angry or combative — usually preceded by the words on the. A volatile, swearing boss is on the warpath. An angry driver swearing at other drivers is on the warpath. It's best to avoid people when they're on the warpath.