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The Collaborative International Dictionary
Unsin

Unsin \Un*sin"\, v. t. [1st pref. un- + sin.] To deprive of sinfulness, as a sin; to make sinless. [Obs.]
--Feltham.

Wiktionary
unsin

vb. 1 (context transitive English) To undo or annul a past sin. 2 (context obsolete transitive English) To deprive (a sin, etc.) of sinfulness; to make sinless.