(of a road, an area, etc.) impossible to travel on or through, especially because it is in bad condition or it has been blocked by something The mountain roads are totally impassable to cars in winter. After the storm, many roads were made impassable by fallen trees. The river formed an impassable barrier for migrating animals. The river’s broad mudflats were thought completely impassable. opposite passable See related entries: Features of roads Word Origin mid 16th cent.: perhaps via French.