The phrase "stopt up" means blocked, obstructed, or congested. The antonyms for this phrase can be open, clear, empty, or unobstructed. When something is "stopt up," it means there is no free movement or proper flow. On the other hand, when something is open or clear, it means that it is free from any blockages. If something is unobstructed, it can be easily seen or accessed without any hindrance. Therefore, it is important to use the right antonym for "stopt up" to convey the intended meaning effectively.