1530s, "to unload" (transitive), from un- (2) "reverse of" + burden (v.). Similar formation in German entbürden. Reflexive sense is recorded from 1580s. Related: Unburdened; unburdening.
unbreakable
unbridled
un-British
unbroken
unbuckle
unburden
unburied
unbutton
uncalled
uncanny
uncap
