The phrase "untie the knot" refers to the act of untying or undoing a knot, which can be both literal and figurative. When we use antonyms for this phrase, we are essentially looking at the opposite of untying the knot, which could mean tightening or securing a knot. Some possible antonyms for "untie the knot" could include "tie the knot," "fasten the knot," "bind the knot," or "secure the knot." These phrases imply a sense of stability, permanence, or commitment, which is the opposite of the freedom and release implied by "untie the knot." Ultimately, the choice of antonym depends on the specific context or situation in which this phrase is being used.