The word "un-tightening" means to loosen or relax. Antonyms for this word would be to tighten, secure, fasten, or strengthen. If you are working with a bolt, you might use a wrench to tighten it, instead of un-tightening it. If you are securing a rope on a boat, you might want to fasten it securely, instead of un-tightening it. When building a structure, you would want to strengthen the support beams, instead of loosening them. Antonyms for the word "un-tightening" are important in many situations, especially those that involve safety and security.