Late 17th century; earliest use found in Afra Behn (?1640–1689), writer. From un- + resolve, probably after unresolved.
unresolve2
/(ˌ)ʌnrɪˈzɒlv/
Now rare
verb
[with object]To resolve against (a course of action upon which one has previously resolved). Also without object: to reverse or go back upon a previous resolution.
Origin
Early 17th century; earliest use found in Henry Wotton (1568–1639), diplomat and writer. From un- + resolve.