The antonyms (opposites) for the word "un-tighten" are many and varied. Some include, loosen, relax, slacken, ease, relieve, unwind, unclench, untangle, and unfurl. These antonyms connote a sense of release or freedom, as opposed to the constrictive nature of being "un-tightened". For instance, when a muscle is "un-tightened," it's hard and contracted, but when it's "relaxed", it's soft and easy. Similarly, when a knot is "un-tightened," it's still a knot, but when it's "untangled", it's free and open. In essence, the antonyms for "un-tighten" evoke a sense of liberation, expansion, and relaxation.