Kenneth G. Wilson (1923). The Columbia Guide to Standard American English. 1993.
unloose, unloosen (vv.)
These Standard weak verbs (principal parts: unloose, unloosed, unloosed; unloosen, unloosened, unloosened) are synonyms meaning to set loose, release, make less tight, or undo, as in He unloosed [unloosened] his tie. See LOOSE.