From The Collaborative International Dictionary of English v.0.48 [gcide]:
Unstrain \Un*strain"\, v. t. [1st pref. un- + strain.]
To relieve from a strain; to relax. --B. Jonson.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
unstrain
v 1: cause to feel relaxed; "A hot bath always relaxes me" [syn:
relax, unstrain, unlax, loosen up, unwind, {make
relaxed}] [ant: strain, tense, tense up]