Stopping up is the act of obstructing or blocking a pathway or space. The antonyms for stopping up include opening up, clearing out, unblocking, and freeing. When we open up a pathway, we make it possible for things to flow freely. Similarly, clearing out a space involves removing any obstacles that may be present, allowing for easy movement. Unblocking involves taking measures to remove any blockages that may be present in a pathway or space. Freeing, on the other hand, involves getting rid of any impediments that may be preventing smooth and unrestricted movement. All of these antonyms represent the opposite of stopping up, facilitating movement and flow instead of impeding it.