The word "un tighten" generally means to loosen or relax something that was previously tightened. Its antonyms, on the other hand, relate to the opposite action of tightening or securing. Some common antonyms for "un tighten" include fasten, tighten, lock, clench, close, tighten up and secure. These words imply the act of making something tight or fixing it firmly in place, leading to reduced movement or flexibility. The opposite of "un tighten" is necessary in many areas of life, from securing a load in transportation to locking doors and windows for safety.