c. 1500, "not plundered," from un- (1) "not" + past participle of spoil (v.). Meaning "not deteriorated" is attested from 1732. A verb unspoil is attested from c. 1400, but it meant "despoil."
unsorted
unsought
unsound
unsparing
unspeakable
unspoiled
unspoken
unsporting
unsportsmanlike
unstable
unstack