The phrase "letting the hook" typically means to release or abandon something, whereas antonyms for this expression would include securing, fastening, or tightening. These antonyms suggest actions that involve keeping something in place or holding onto it, as opposed to letting it go. For instance, if a fisherman is not "letting the hook" go, they might be tightening their grip on the fishing line or reeling in their catch. Similarly, if someone is "letting the hook" on a particular project or task, antonyms would include ensuring its completion or committing to seeing it through to the end.