The Collaborative International Dictionary
Untighten \Un*tight"en\, v. t. [1st pref. un- + tighten.] To make less tight or tense; to loosen.
Wiktionary
vb. (context transitive English) To loosen; to make something less tight.
Usage examples of "untighten".
With the other, he tightened and untightened the screws that moved his fingers.
Here and there he loosened a screw, yonder he untightened a bolt or two.
In the classroom, on the rare occasions she sat in her chair, she would continually rise and fall behind her desk, as if she were tightening and untightening her buttocks.
Slowly he released his sword, tightening and untightening his grip on the hilt angrily.
Carialle also gave her own hardware a good going over, to make certain the boffins in the repair bay at SSS-900-C, the last space station they had visited, hadn't left any screws untightened when they had examined her innards to install a ton of new memory.