Source : Webster's Revised Unabridged Dictionary (1913)

Unstrain \Un*strain"\, v. t. [1st pref. un- + strain.]
   To relieve from a strain; to relax. --B. Jonson.

Source : WordNet®

unstrain
     v : cause to feel relaxed; "A hot bath always relaxes me" [syn:
         {relax}, {unlax}, {loosen up}, {unwind}, {make relaxed}]
         [ant: {tense}, {tense}]