1To prise or force open (a person's tightly closed hand, a grip, etc.); to cause to open; = "unclench".
2To relax or open (one's tightly closed hand, fist, etc.); = "unclench".
3 rare And reflexive. Of a tightly closed hand, fist, etc.: to relax; to open; = "unclench".
4To loosen (a nail, especially one used to attach a horseshoe) by prising up the point, when this has been bent over or flattened to secure it. Formerly also: †to remove the nails from (a horseshoe) (obsolete).
Late 16th century; earliest use found in John Florio (1553–1625), author and teacher of languages. From un- + clinch.